Optical display systems and methods

ABSTRACT

Optical display systems and methods are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to U.S.Provisional Patent Application Ser. No. 60/553,894, filed Mar. 16, 2004,and entitled “Optical Display Systems and Methods”, the entire contentsof which are hereby incorporated by reference. This application is acontinuation-in-part of, and claims priority under 35 U.S.C. §120 to,U.S. patent application Ser. No. 10/735,498, filed Dec. 12, 2003 nowU.S. Pat. No. 7,166,871, and entitled “Light Emitting Systems”, U.S.patent application Ser. No. 10/794,244, filed Mar. 5, 2004, and entitled“Light Emitting Device Methods”, and U.S. patent application Ser. No.10/794,452, filed Mar. 5, 2004 now U.S. Pat. No. 7,074,631, and entitled“Light Emitting Device Methods”, the entire contents of each of which ishereby incorporated by reference.

INCORPORATION BY REFERENCE

This application incorporates by reference the following U.S.Provisional Patent Applications: 60/462,889, filed Apr. 15, 2003;60/474,199, filed May 29, 2003; 60/475,682, filed Jun. 4, 2003;60/503,653, filed Sep. 17, 2003; 60/503,654 filed Sep. 17, 2003;60/503,661, filed Sep. 17, 2003; 60/503,671, filed Sep. 17, 2003;60/503,672, filed Sep. 17, 2003; 60/513,807, filed Oct. 23, 2003; and60/514,764, filed Oct. 27, 2003. This application also incorporates byreference the following U.S. Patent Applications: U.S. Ser. No.10/723,987 entitled “Light Emitting Devices,” and filed Nov. 26, 2003;U.S. Ser. No. 10/724,004, entitled “Light Emitting Devices,” and filedNov. 26, 2003; U.S. Ser. No. 10/724,033, entitled “Light EmittingDevices,” and filed Nov. 26, 2003; U.S. Ser. No. 10/724,006, entitled“Light Emitting Devices,” and filed Nov. 26, 2003; U.S. Ser. No.10/724,029, entitled “Light Emitting Devices,” and filed Nov. 26, 2003;U.S. Ser. No. 10/724,015, entitled “Light Emitting Devices,” and filedNov. 26, 2003; and U.S. Ser. No. 10/724,005, entitled “Light EmittingDevices,” and filed Nov. 26, 2003.

TECHNICAL FIELD

The invention relates to optical display systems and methods.

BACKGROUND

A light emitting diode (LED) often can provide light in a more efficientmanner than an incandescent light source and/or a fluorescent lightsource. The relatively high power efficiency associated with LEDs hascreated an interest in using LEDs to displace conventional light sourcesin a variety of lighting applications. For example, in some instancesLEDs are being used as traffic lights and to illuminate cell phonekeypads and displays.

Typically, an LED is formed of multiple layers, with at least some ofthe layers being formed of different materials. In general, thematerials and thicknesses selected for the layers determine thewavelength(s) of light emitted by the LED. In addition, the chemicalcomposition of the layers can be selected to try to isolate injectedelectrical charge carriers into regions (commonly referred to as quantumwells) for relatively efficient conversion to optical power. Generally,the layers on one side of the junction where a quantum well is grown aredoped with donor atoms that result in high electron concentration (suchlayers are commonly referred to as n-type layers), and the layers on theopposite side are doped with acceptor atoms that result in a relativelyhigh hole concentration (such layers are commonly referred to as p-typelayers).

A common approach to preparing an LED is as follows. The layers ofmaterial are prepared in the form of a wafer. Typically, the layers areformed using an epitaxial deposition technique, such as metal-organicchemical vapor deposition (MOCVD), with the initially deposited layerbeing formed on a growth substrate. The layers are then exposed tovarious etching and metallization techniques to form contacts forelectrical current injection, and the wafer is subsequently sectionedinto individual LED chips. Usually, the LED chips are packaged.

During use, electrical energy is usually injected into an LED and thenconverted into electromagnetic radiation (light), some of which isextracted from the LED.

SUMMARY

The invention relates to optical display systems and methods.

In one aspect, the invention features an optical display system thatincludes a microdisplay having a surface and a light emitting device.The light emitting device includes a multi-layer stack of materialsincluding a light generating region, and a first layer supported by thelight generating region. A surface of the first layer is configured sothat the light generated by the light generating region can emerge fromthe light emitting device via the surface of the first layer. The ratioof the aspect ratio of the surface of the microdisplay and the aspectratio of the surface of the first layer is at most about two.

In another aspect, the invention features an optical display system thatincludes a microdisplay, a light emitting device, and at least oneoptical component disposed along an optical path from the microdisplayto the light emitting device. The microdisplay, the light emittingdevice and the optical component are positioned so that, during use, animage plane of the system does not coincide with a surface of themicrodisplay illuminated by light emitted by the light emitting device.

In a further aspect, the invention features a method of illuminating amicrodisplay using a plurality of light emitting devices. The methodincludes activating the plurality of light emitting devices so that atleast one of the plurality of light emitting devices has an activationtime that is different from an activation time of at least one of theother light emitting devices in the plurality of light emitting devices.

In one aspect, the invention features an optical display system thatincludes a LED and a cooling system that is configured so that, duringuse, the cooling system regulates a temperature of the light emittingdiode.

In another aspect, the invention features a light emitting device thatincludes a multi-layer stack of materials. The multi-layer stack ofmaterials includes a light generating region, and a first layersupported by the light generating region. The surface of the first layeris configured so that the light generated by the light generating regioncan emerge from the light emitting device via the surface of the firstlayer. The surface of the first layer has a contact region configured sothat, during use, at most about 20% an area of a microdisplayilluminated by the light emitting diode comprises dark spots formed dueto the contact region.

In a further aspect, the invention features an optical display systemthat includes a plurality of light emitting diodes, a microdisplay, atleast one optical component disposed along an optical path from themicrodisplay to the light emitting diode, and a beam aggregation deviceconfigured to combine light generated by the plurality of light emittingdiodes.

In an additional aspect, the invention features a system that includes alight emitting device. The light emitting device includes a rectangularmulti-layer stack of materials including a light generating region, anda first layer supported by the light generating region. The surface ofthe first layer is configured so that light generated by the lightgenerating region can emerge from the light emitting device via thesurface of the first layer The light emitting device also includes atleast one electrical contact pad disposed along an edge of the surfaceof the first layer, and a package. The package includes a plurality ofplated regions for providing electrical contact to the light emittingdevice, a plurality of castellations to provide electrical contact fromone surface of the package to the opposite surface of the package, and aplurality of wire bonds connected between the plated regions and the atleast one electrical contact pad.

In one aspect, the invention features a device that includes a materialbody designed for use in an electronic device, and a contact structuresupported by the surface of the material body. The contact structureincludes a patterned conductive layer and a patterned insulating layer.The patterned insulating layer is disposed between the interior portionof the patterned conductive layer and the surface of the material body.The insulating layer is patterned such that the conductive layer extendspast all edges of the insulating layer to form an electrical contact tothe material body.

In another aspect, the invention features a device that includes asemiconductor die. The device also includes a conductive pad disposedalong one side of a surface layer of the semiconductor die and anotherconductive pad disposed along the opposite side of the surface layer ofthe semiconductor die. The device further includes a plurality ofconductive contacts in electrical contact with at least one of theconductive pads and extending from at least one of the conductive padstoward a central area of the semiconductor die. The device also includesan insulating layer disposed between an interior portion of at leastsome of the plurality of conductive contacts and the top layer of thesemiconductor die.

In a further aspect, the invention features a device that includes arectangular light emitting diode (LED). The device also includes aconductive pad disposed along one side of a surface layer of therectangular LED and another conductive pad disposed along the oppositeside of the surface layer of the rectangular LED. The device furtherincludes a plurality of conductive contacts in electrical contact withat least one of the conductive pads and extending from at least one ofthe conductive pads toward a central area of the rectangular LED.

In an additional aspect, the invention features a device that includes astack of materials and a contact structure disposed on the surface ofthe stack of materials. The contact structure includes a patternedconductive layer and a patterned insulating layer disposed between thepatterned conductive layer and the surface of the stack of materials.The patterned conductive layer and the patterned insulating layer areconfigured such that during use a voltage drop across the patternedconductive layer is about the same at a plurality of segments disposedalong the length of the patterned conductive layer.

In yet another aspect, the invention features a device that includes astack of materials and a contact structure disposed on the surface ofthe stack of materials. The contact structure includes a patternedconductive layer configured such that during use a heat generationacross the patterned conductive layer is about the same at a pluralityof segments disposed across the patterned conductive layer.

Embodiments can feature one or more of the following advantages.

In certain embodiments, a light-emitting system can include an LEDand/or a relatively large LED chip that can exhibit relatively highlight extraction.

In some embodiments, a light-emitting system can include an LED and/or arelatively large LED chip that can exhibit relatively high surfacebrightness, relatively high average surface brightness, relatively lowneed for heat dissipation or relatively high rate of heat dissipation,relatively low etendue and/or relatively high power efficiency.

In certain embodiments, a light-emitting system can include an LEDand/or a relatively large LED chip that can be designed so thatrelatively little light emitted by the LED/LED chip is absorbed bypackaging.

In some embodiments, a light-emitting system can include a packaged LED(e.g., a relatively large packaged LED) that can be prepared withoutusing an encapsulant material. This can result in a packaged LED thatavoids certain problems associated with the use of certain encapsulantmaterials, such as reduced performance and/or inconsistent performanceas a function of time, thereby providing a packaged LED that can exhibitrelatively good and/or reliable performance over a relatively longperiod of time.

In certain embodiments, a light-emitting system can include an LED(e.g., a packaged LED, which can be a relatively large packaged LED)that can have a relatively uniform coating of a phosphor material.

In some embodiments, a light-emitting system can include an LED (e.g., apackaged LED, which can be a relatively large packaged LED) that can bedesigned to provide a desired light output within a particular angularrange (e.g., within a particular angular range relative to the LEDsurface normal).

In some embodiments, a light-emitting system can include an LED and/or arelatively large LED chip that can be prepared by a process that isrelatively inexpensive.

In certain embodiments, a light-emitting system can include an LEDand/or a relatively large LED chip that can be prepared by a processthat can be conducted on a commercial scale without incurring costs thatrender the process economically unfeasible.

In some embodiments, using a rectangular shape for an LED (compared, forexample, to a square) can provide certain advantages. The advantages caninclude one or more of the following. The rectangular LED can allow agreater number of wire bonds per unit area increasing the power that canbe input into the LED. The rectangular shape can be chosen to match aparticular aspect ratio of a pixel or microdisplay, thus, eliminatingthe need for complex beam shaping optics. The rectangular shape can alsoimprove heat dissipation from the LED and reduce the likelihood offailure due to the device overheating. Also, because the cross sectionof an individual LEDs cut from a wafer is only slightly larger than thelight-emitting surface area of the LED, many individual, and separatelyaddressable LEDs can be packed closely together in an array. If one LEDdoes not function (e.g., due to a large defect), then it does notsignificant diminish the performance of the array because the individualdevices are closely packed.

Features and advantages of the invention are in the description,drawings and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of a light emitting system.

FIG. 2A-2D are schematic representations of optical display systems.

FIG. 3 is a schematic representation of an optical display system.

FIG. 4A is a schematic representation of a top view of an LED.

FIG. 4B is a schematic representation of an optical display system.

FIG. 5 is a schematic representation of an optical display system.

FIG. 6 is a schematic representation of an optical display system.

FIG. 7 is a schematic representation of an optical display system.

FIGS. 8A and 8B are schematic representations of an optical displaysystem.

FIG. 9 is a schematic representation of an optical display system.

FIG. 10 is a schematic representation of an optical display system.

FIG. 11 is a schematic representation of an optical display system.

FIG. 12 is a cross-sectional view of an LED with a patterned surface.

FIG. 13 is a top view the patterned surface of the LED of FIG. 2.

FIG. 14 is a graph of an extraction efficiency of an LED with apatterned surface as function of a detuning parameter.

FIG. 15 is a schematic representation of the Fourier transformation of apatterned surface of an LED.

FIG. 16 is a graph of an extraction efficiency of an LED with apatterned surface as function of nearest neighbor distance.

FIG. 17 is a graph of an extraction efficiency of an LED with apatterned surface as function of a filling factor.

FIG. 18 is a top view a patterned surface of an LED.

FIG. 19 is a graph of an extraction efficiency of LEDs with differentsurface patterns.

FIG. 20 is a graph of an extraction efficiency of LEDs with differentsurface patterns.

FIG. 21 is a graph of an extraction efficiency of LEDs with differentsurface patterns.

FIG. 22 is a graph of an extraction efficiency of LEDs with differentsurface patterns.

FIG. 23 is a schematic representation of the Fourier transformation twoLEDs having different patterned surfaces compared with the radiationemission spectrum of the LEDs.

FIG. 24 is a graph of an extraction efficiency of LEDs having differentsurface patterns as a function of angle.

FIG. 25 is a side view of an LED with a patterned surface and a phosphorlayer on the patterned surface.

FIG. 26 is a cross-sectional view of a multi-layer stack.

FIG. 27 is a cross-sectional view of a multi-layer stack.

FIG. 28 is a cross-sectional view of a multi-layer stack.

FIG. 29 is a cross-sectional view of a multi-layer stack.

FIG. 30 depicts a side view of a substrate removal process.

FIG. 31 is a partial cross-sectional view of a multi-layer stack.

FIG. 32 is a partial cross-sectional view of a multi-layer stack.

FIG. 33 is a partial cross-sectional view of a multi-layer stack.

FIG. 34 is a partial cross-sectional view of a multi-layer stack.

FIG. 35 is a partial cross-sectional view of a multi-layer stack.

FIG. 36 is a partial cross-sectional view of a multi-layer stack.

FIG. 37 is a partial cross-sectional view of a multi-layer stack.

FIG. 38 is a partial cross-sectional view of a multi-layer stack.

FIG. 39 is a partial cross-sectional view of a multi-layer stack.

FIG. 40 is a partial cross-sectional view of a multi-layer stack.

FIG. 41 is a partial cross-sectional view of a multi-layer stack.

FIG. 42 is a partial cross-sectional view of a multi-layer stack.

FIG. 43 is a partial cross-sectional view of a multi-layer stack.

FIG. 44 is a partial cross-sectional view of a multi-layer stack.

FIG. 45A is a perspective view of an LED.

FIG. 45B is a top view of an LED.

FIG. 46A is a top view of an LED.

FIG. 46B is a partial cross-sectional view of an LED.

FIG. 46C is an equivalent circuit diagram.

FIG. 47A is a top view of an LED.

FIG. 47B is an equivalent circuit diagram.

FIG. 48A is a top view of an LED.

FIG. 48B is an equivalent circuit diagram.

FIG. 49A is a top view of an LED.

FIG. 49B is a partial cross-sectional view of an LED.

FIG. 49C is a partial cross-sectional view of an LED.

FIG. 50 is a graph of junction current density.

FIG. 51A is a top view of a multi-layer stack.

FIG. 51B is a partial cross-sectional view of an LED.

FIG. 52 is a view of a contact.

FIG. 53 is a diagram of a packaged LED.

FIG. 54 is a diagram of a packaged LED and a heat sink.

FIG. 55 is a graph of resistance.

FIG. 56 is a graph of junction temperature.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 is a schematic representation of a light-emitting system 50 thathas an array 60 of LEDs 100 incorporated therein. Array 60 is configuredso that, during use, light that emerges from LEDs 100 (see discussionbelow) emerges from system 50 via surface 55.

Examples of light-emitting systems include projectors (e.g., rearprojection projectors, front projection projectors), portable electronicdevices (e.g., cell phones, personal digital assistants, laptopcomputers), computer monitors, large area signage (e.g., highwaysignage), vehicle interior lighting (e.g., dashboard lighting), vehicleexterior lighting (e.g., vehicle headlights, including color changeableheadlights), general lighting (e.g., office overhead lighting), highbrightness lighting (e.g., streetlights), camera flashes, medicaldevices (e.g., endoscopes), telecommunications (e.g. plastic fibers forshort range data transfer), security sensing (e.g. biometrics),integrated optoelectronics (e.g., intrachip and interchip opticalinterconnects and optical clocking), military field communications(e.g., point to point communications), biosensing (e.g. photo-detectionof organic or inorganic substances), photodynamic therapy (e.g. skintreatment), night-vision goggles, solar powered transit lighting,emergency lighting, airport runway lighting, airline lighting, surgicalgoggles, wearable light sources (e.g. life-vests). An example of a rearprojection projector is a rear projector television. An example of afront projection projector is a projector for displaying on a surface,such as a screen or a wall. In some embodiments, a laptop computer caninclude a front projection projector.

Typically, surface 55 is formed of a material that transmits at leastabout 20% (e.g., at least about 30%, at least about 40%, at least about50%, at least about 60%, at least about 70%, at least about 80%, atleast about 90%, at least about 95%) of the light that emerges from LEDs100 and impinges on surface 55. Examples of materials from which surface55 can be formed include glass, silica, quartz, plastic, and polymers.

In some embodiments, it may be desirable for the light that emerges(e.g., total light intensity, light intensity as a function ofwavelength, and/or peak emission wavelength) from each LED 100 to besubstantially the same. An example is time-sequencing of substantiallymonochromatic sources (e.g. LEDs) in display applications (e.g., toachieve vibrant full-color displays). Another example is intelecommunications where it can be advantageous for an optical system tohave a particular wavelength of light travel from the source to thelight guide, and from the light guide to the detector. A further exampleis vehicle lighting where color indicates signaling. An additionalexample is in medical applications (e.g., photosensitive drug activationor biosensing applications, where wavelength or color response can beadvantageous).

In certain embodiments, it may be desirable for the light that emerges(e.g., total light intensity, light intensity as a function ofwavelength, and/or peak emission wavelength) from at least some of LEDs100 to be different from the light that emerges (e.g., total lightintensity, light intensity as a function of wavelength, and/or peakemission wavelength) from different LEDs 100. An example is in generallighting (e.g., where multiple wavelengths can improve the colorrendering index (CRI)). CRI is a measurement of the amount of colorshift that objects undergo when lighted by the light-emitting system ascompared with the color of those same objects when seen under areference lighting system (e.g., daylight) of comparable correlatedtemperature. Another example is in camera flashes (e.g., wheresubstantially high CRI, such as substantially close to the CRI ofnoontime sunlight, is desirable for a realistic rendering of the objector subject being photographed). A further example is in medical devices(e.g., where substantially consistent CRI is advantageous for tissue,organ, fluid, etc. differentiation and/or identification). An additionalexample is in backlighting displays (e.g., where certain CRI white lightis often more pleasing or natural to the human eye).

Although depicted in FIG. 1 as being in the form of an array, LEDs 100can be configured differently. As an example, in some embodiments,system 50 includes a single LED 100. As another example, in certainembodiments, the array is curved to help angularly direct the light fromvarious sources onto the same point (e.g., an optic such as a lens). Asa further example, in some embodiments, the array of devices ishexagonally distributed to allow for close-packing and high effectivesurface brightness. As an additional example, in certain embodiments,the devices are distributed around a mirror (e.g., a dichroic mirror)that combines or reflects light from the LEDs in the array.

In FIG. 1 the light that emerges from LEDs 100 is shown as travelingdirectly from LEDs 100 to surface 55. However, in some embodiments, thelight that emerges from LEDs 100 can travel an indirect path from LEDs100 to surface 55. As an example, in some embodiments, system 50includes a single LED 100. As another example, in certain embodiments,light from LEDs 100 is focused onto a microdisplay (e.g., onto a lightvalve such as a digital light processor (DLP) or a liquid crystaldisplay (LCD)). As a further example, in some embodiments, light isdirected through various optics, mirrors or polarizers (e.g., for anLCD). As an additional example, in certain embodiments, light isprojected through primary or secondary optics, such as, for example, alens or a set of lenses.

FIG. 2A shows an optical display system 1100 (see discussion above)including a non-Lambertian LED 1110 (see discussion below), a lens 1120and a microdisplay 1130. LED 1110 is spaced a distance L1 from lens1120, and microdisplay 1130 is spaced a distance L2 from lens 1120.Distances L1 and L2 are selected so that, for light emitted by LED 1110that impinges on lens 1120, the image plane of lens 1120 coincides withthe surface of microdisplay 1130 on which the light emitted by LED 1110impinges.

With this arrangement, system 1100 can use the light emitted by LED 1110to relatively efficiently illuminate the surface of microdisplay 1130with the shape of the surface of LED 1110 that emits light being aboutthe same as the shape of the surface of 1130 that is illuminated by thelight emitted by LED 1110. For example, in some embodiments, the ratiothe aspect ratio of LED 1110 to the aspect ratio of microdisplay 1130can be from about 0.5 to about 2 (e.g., from about 9/16 to about 16/9,from about 3/4 to about 4/3, about 1). The aspect ratio of microdisplay1130 can be, for example, 1920×1080, 640×480, 800×600, 1024×700,1024×768, 1024×720, 1280×720, 1280×768, 1280×960, or 1280×1064.

In general, the surface of microdisplay 1130 and/or the surface of LED1110 can have any desired shape. Examples of such shapes include square,circular, rectangular, triangular, trapezoidal, and hexagonal.

In some embodiments, an optical display system can relativelyefficiently illuminate the surface of microdisplay 1130 without a lensbetween LED 1110 and microdisplay 1130 while still having the shape ofthe surface of LED 1110 that emits light being about the same as theshape of the surface of 1130 that is illuminated by the light emitted byLED 1110. For example, FIG. 2B shows a system 1102 in which a square LED1110 is imaged onto a square microdisplay 1130 without having a lensbetween LED 1110 and microdisplay 1130. As another example, FIG. 2Cshows an optical display system 1104 in which a rectangular LED 1110 canbe imaged onto a rectangular microdisplay 1130 (with a similarlyproportioned aspect ratio) without having a lens between LED 1110 andmicrodisplay 1130.

In certain embodiments, an anamorphic lens can be disposed between LED1110 and microdisplay 1130. This can be desirable, for example, when theaspect ratio of LED 1110 is substantially different from the aspectratio of microdisplay 1130. As an example, FIG. 2D shows a system 1106that includes LED 1110 having a substantially square shaped surface,microdisplay 1130 having a substantially rectangular shaped surface(e.g., an aspect ratio of about 16:9 or about 4:3), and an anamorphiclens 1120 disposed between LED 1110 and microdisplay 1130. In thisexample, anamorphic lens 1120 can be used to convert the shape of thelight emitted by LED 1110 to substantially match the shape of thesurface of microdisplay 1130. This can enhance the efficiency of thesystem by increasing the amount of light emitted by the surface of LED1110 that impinges upon the surface of microdisplay 1130.

FIG. 3 shows an optical display system 1200 including LED 1110, lens1120, and microdisplay 1130. The light emitting surface of LED 1110 hascontact regions to which electrical leads 1115 are attached (seediscussion below). LED 1110 is spaced a distance L3 from lens 1120, andmicrodisplay 1130 is spaced a distance L4 from lens 1120. Leads 1115block light from being emitted from the contact regions of LED 1110. Ifthe plane of the surface of microdisplay 1130 on which the light emittedby LED 1110 impinges coincides with the image plane of lens 1120, a setof dark spots 1202 corresponding to the contact region of the lightemitting surface of LED 1110 can appear on this surface of microdisplay1130. To reduce the area of this surface of microdisplay 1130 that iscovered by the dark spots, distances L3 and L4 are selected so that, forlight emitted by LED 1110 that impinges on lens 1120, the image plane oflens 1120 does not coincide with the plane of the surface ofmicrodisplay 1130 on which the light emitted by LED 1110 impinges (i.e.,there exists a distance, ΔL, between the image plane of lens 1120 andthe plane of the surface of microdisplay 1130 on which the light emittedby LED 1110 impinges). With this arrangement, the light from LED 1110 isdefocused in the plane of the surface of microdisplay 1130 on which thelight emitted by LED 1110 impinges, and the resulting intensity of lightis more uniform on this surface of microdisplay 1130 than in the imageplane of lens 1120. The total distance between the LED and themicrodisplay 1130 can be represented as the distance between the LED1110 and the image plane 1120 (L5) plus the distance, ΔL. In general, asΔL is increased by increasing the distance between the LED 1110 and themicrodisplay 1130, the intensity of dark spots decreases but theintensity of light emitted by LED 1110 that impinges on the surface ofmicrodisplay 1130 decreases. Alternately, when the microdisplay istranslated such that the distance between the LED 1110 and themicrodisplay 1130 is decreased, the intensity is greater than theintensity at the image plane, but the microdisplay may be only partiallyilluminated. In some embodiments, the absolute value of ΔL/L5 is fromabout 0.00001 to about 1 (e.g., from about 0.00001 to about 0.1, fromabout 0.00001 to about 0.01, from about 0.00001 to about 0.001), or fromabout 0.00001 to about 0.0001) In some embodiments, multiple LEDs may beused to illuminate a single microdisplay (e.g., a 3×3 matrix of LEDs).Such a system can be desirable because, when multiple LEDs are arrangedto illuminate a single microdisplay, if one LEDs fails, the system wouldstill be useable (however a dark spot may occur due to the absence oflight from the particular LED). If multiple LEDs are used to illuminatea single microdisplay, the optical system can be configures so that darkspots do not appear on the surface of the microdisplay. For example, themicrodisplay can be translated outside of the image plane such that thearea between the LEDs does not result in a dark spot.

In some embodiments, the intensity of dark spots on the surface ofmicrodisplay 1130 can be reduced by appropriately configuring thecontact region of the surface of LED 1110. For example, FIG. 4A shows atop view of an LED 1110 with a contact region disposed around theperimeter of LED 1110. With this arrangement, with or without thepresence of a lens (with or without defocusing), the optical displaysystem can be configured (e.g., by properly sizing the area of thesurface of microdisplay 1130) so that the intensity of the dark spotscreated by the contact region of the surface of LED 1110 on surface 1130is relatively small. This approach may be used with systems that includemultiple LEDs (e.g., a 3×3 matrix of LEDs).

As another example, FIG. 4B shows an optical display system 300 thatincludes LED 1110 and microdisplay 1130. LED 1110 includes a contactregion formed by leads 1115 that is selected so that dark spots 1202appear at a region not imaged on the surface of microdisplay 1130. Inthis example, the surface of microdisplay 1130 can be located at theimage plane of lens 1120 because the dark spots fall outside of the areaimaged on the microdisplay at the image plane of lens 1120. If the shapeof LED 1110 is matched to the shape of microdisplay 1130, leads 1115 canbe disposed, for example, on the surface of LED 1110 around itsperimeter. In this example, the area inside the contact region ofsurface 1110 matches (e.g., the aspect ratio is similar) to the surfaceof microdisplay 1130. This approach may be used with systems thatinclude multiple LEDs (e.g., a 3×3 matrix of LEDs).

As a further example, FIG. 5 shows an optical display system 1700 thatincludes LED 1110 and microdisplay 1130. LED 1110 also includes acontact region formed by leads 1115 and a homogenizer 1702 (alsoreferred to as a light tunnel or light pipe) that guides light emittedfrom LED 1110 to a lens 1120. Total internal reflection of the lightemitted by LED 1110 off the inside surfaces of homogenizer 1702 cangenerate a substantially uniform output distribution of light and canreduce the appearance of dark spots caused by leads 1115 so thatmicrodisplay 1130 is substantially uniformly illuminated by LED 1110(e.g., an image generated in an image plane 1131 is substantiallyuniform).

Optionally, system 1700 can include one or more additional opticalcomponents. For example, in some embodiments, optical display system1700 can also include a lens disposed in the path prior to thehomogenizer to focus light into the homogenizer. In certain embodiments,the aspect ratio of the aperture of homogenizer 1702 matches that of LED1110 such that when LED 1110 is mounted in close proximity tohomogenizer 1702, additional lenses may not be necessary or such thatmore efficient coupling of light into homogenizer 1702 is possible witha lens prior to homogenizer 1702.

As an additional example, FIG. 6 shows an optical display system 1710that includes LED 1110 and microdisplay 1130. LED 1110 also includes acontact region formed by leads 1115 and a set of multiple lenses 1712that are disposed between LED 1110 and lens 1120. Lenses 1712 can varyin size, shape, and number. For example, the number and size of lenses1712 can be proportional to the cross-sectional area of LED 1110. Insome embodiments, lenses 1712 include a set of between about 1 and about100 lenses with sizes varying of, for example, from about 1 mm to about10 cm. The light emitted by LED 1110, enters lenses 1712 and isrefracted. Since the surfaces of lenses 1712 are curved, the lightrefracts at different angles causing the beams emerging from lenses 1712to overlap. The overlapping of the beams reduces the appearance of darkspots caused by leads 1115 so that microdisplay 1130 is substantiallyuniformly illuminated by LED 1110 (e.g., an image generated in an imageplane 1131 is substantially uniform).

While optical display systems have been described as including a singlelens, in some embodiments, multiple lenses can be used. Further, incertain embodiments, one or more optical components other than lens(es)can be used. Examples of such optical components include mirrors,reflectors, collimators, beam splitters, beam combiners, dichroicmirrors, filters, polarizers, polarizing beam splitters, prisms, totalinternal reflection prisms, optical fibers, light guides and beamhomogenizers. The selection of appropriate optical components, as wellas the corresponding arrangement of the components in the system, isknown to those skilled in the art.

Moreover, although optical display systems have been described asincluding one non-Lambertian LED, in some embodiments, more than onenon-Lambertian LED can be used to illuminate microdisplay 1130. Forexample, FIG. 7 shows a system 1500 that includes a blue LED 1410 (anLED with a dominant output wavelength from about 450 to about 480 nm), agreen LED 1420 (an LED with a dominant output wavelength from about 500to about 550 nm), and a red LED 1430 (an LED with a dominant outputwavelength from about 610 to about 650 μm) which are in opticalcommunication with the surface of microdisplay 1130. LEDS 1410, 1420,and 1430 can be arranged to be activated simultaneously, in sequence orboth. In other embodiments, at least some of the LEDs may be in opticalcommunication with separate microdisplay surfaces.

In some embodiments, LEDs 1410, 1420, and 1430 are activated insequence. In such embodiments, a viewer's eye generally retains andcombines the images produced by the multiple colors of LEDs. Forexample, if a particular pixel (or set of pixels) or microdisplay (orportion of a microdisplay) of a frame is intended to be purple in color,the surface of the microdisplay can be illuminated with red LED 1430 andblue LED 1410 during the appropriate portions of a refresh cycle. Theeye of a viewer combines the red and the blue and “sees” a purplemicrodisplay. In order for a human not to notice the sequentialillumination of the LEDs, a refresh cycle having an appropriatefrequency (e.g., a refresh rate greater than 120 Hz) can be used.

LEDs 1410, 1420 and 1430 may have varying intensities and brightness.For example, green LED 1420 may have a lower efficiency than red LED1430 or blue LED 1410. Due to a particular LED (e.g., green LED 1420)having a lower efficiency, it can be difficult to illuminate the surfaceof the microdisplay with a sufficiently high brightness of the color oflight (e.g., green) emitted by the relatively low efficiency LED (e.g.,LED 1420). To compensate for this disparity in efficiency (to produce animage that is not distorted due to the difference in light brightness),the activation cycles for the multiple LEDs can be adjusted. Forexample, the least efficient LED may be allocated a longer activationtime (i.e., on for a longer period of time) than the more efficientLEDs. In a particular example, for a red/green/blue projection systeminstead of a 1/3:1/3:1/3 duty cycle allocation, the cycle may be in theratio of 1/6:2/3:1/6 (red:green:blue). In another example, the cycle maybe in the ratio of 0.25:0.45:0.30 (red:green:blue). In other examples,the duty cycle dedicated to the activation of the green LED may befurther increased. For example, the duty cycle dedicated to imaging thegreen LED 1420 can be greater than about 40% (e.g., greater than about45%, greater than about 50%, greater than about 60%, greater than about70%, greater than about 80%, greater than about 90%). In someembodiments, the duty cycle for each LED is different. As an example,the duty cycle for red LED 1430 can be greater than the duty cycle forblue LED 1410. While systems have been described in which the activationcycle is selected based on the intensity and/or brightness of an LED, insome systems the activation time of an LED may be selected based on oneor more other parameters. In some examples, the activation time of theleast efficient light emitting device is at least about 1.25 times(e.g., at least about 1.5 times, at least about 2 times, at least about3 times) the activation time of another light emitting device.

FIG. 8A shows an embodiment of a liquid crystal display (LCD) basedoptical display system 1720 including blue LED 1410, green LED 1420, andred LED 1430 (e.g., as described above) which are in opticalcommunication with the surface of associated LCD panels 1728, 1730, and1732. Optical display system 1720 also includes lenses 1722, 1724, and1726 in a corresponding optical path between LEDs 1410, 1420, and 1430and associated LCD panels 1728, 1730, and 1732. Lenses 1722, 1724, and1726 focus the light onto associated LCD panels 1728, 1730, and 1732.Optical display system 1720 further includes a device 1734 (e.g., anx-cube) that combines multiple beams of light from LCD panels 1728,1730, and 1732 into a single beam 1736 (indicated by arrows) that can bedirected to a projection lens 1735 or other display Optionally, opticaldisplay system 1720 can include a polarizer that transmits a desiredpolarization (e.g. the ‘p’ polarization) while reflecting anotherpolarization (e.g. the ‘s’ polarization). The polarizer can be disposedin the path between LEDs 1410, 1420, and 1430 and associated lenses1722, 1724, and 1726, between lenses 1722, 1724, and 1726 and theassociated LCD panels 1728, 1730, and 1732, or in other locations alongthe optical path. As shown in FIG. 8B, in some embodiments the aspectratio of an LED (e.g., LED 1430) can be matched to the aspect ratio ofthe microdisplay (e.g., microdisplay 1732) as described above.

FIG. 9 shows an embodiment of a digital light processor (DLP) basedoptical display system 1750 including blue LED 1410, green LED 1420, andred LED 1430 (as described above) which are each in opticalcommunication with associated lenses 1722, 1724, and 1726 (as describedabove). Light emitted from LEDs 1410, 1420, and 1430 passes through theassociated lenses 1722, 1724, and 1726 and is collected by a device 1734(e.g., an x-cube) that combines multiple beams of light emitted by LEDs1410, 1420, and 1430 into a single beam that can be directed to a totalinternal reflection (TIR) prism 1752. For example, the light emergingfrom x-cube 1734 can be directed to TIR prism 1752 by a mirror 1754 orother device such as a light guide. TIR prism 1752 reflects light anddirects the light to a DLP panel 1756. DLP panel 1756 includes aplurality of mirrors that can be actuated to generate a particularimage. For example, a particular mirror can either reflect light 1760(indicated by arrows) such that the light is directed to a projection1755 or can cause the light to be reflected away from projection lens1755. The combination of the LEDs 1410, 1420, and 1430 and DLP panel1756 allow greater control of the signal. For example, the amount ofdata sent to DLP panel 1756 can be reduced (allowing greater switchingfrequency) by switching on and off LEDs 1410, 1420, and 1430 in additionto the mirrors in DLP panel 1756. For example, if no red is needed in aparticular image, red LED 1430 can be switched off eliminating the needto send a signal to DLP 1752 to switch the associated mirror. Theability to modulate the LEDs can improve for example color quality,image quality, or contrast.

FIG. 10 shows a particular embodiment of a liquid crystal on silicon(LCOS) based optical display system 1770 including blue LED 1410, greenLED 1420, and red LED 1430 (as described above) which are each inoptical communication with an associated polarizing beam splitter 1774,1778, and 1782. Light emitted from LEDs 1410, 1420, and 1430 passesthrough the associated polarizing beam splitters 1774, 1778, and 1782and is projected onto an associated LCOS panel 1772, 1776, or 1780.Since LCOS panels 1772, 1776, and 1780 are not sensitive to allpolarizations of light, the polarizing beam splitters 1774, 1778, and1782 polarize the light to a particular polarization (e.g., bytransmitting a desired polarization (e.g., the ‘p’ polarization) whilereflecting another polarization (e.g., the ‘s’ polarization) thepolarization of some light and pass other polarizations) based on thesensitivity of LCOS panels 1772, 1776, and 1780. The light reflectedfrom LCOS panels 1772, 1776, and 1780 is collected by a device 1734(e.g., an x-cube) that combines the beams of light from the multipleLCOS panels 1772, 1776, and 1780 to generate a beam 1790 (indicated byarrows) that is directed to a projection lens 1795.

While in the above examples, the optical display system includes red,green, and blue light emitting devices, other colors and combinationsare possible. For example, the system need not have only three colors.Additional colors such as yellow may be included and allocated a portionof the duty cycle. Alternately, multiple LEDs having different dominantwavelengths may be optically combined to produce a resulting color. Forexample, a blue-green LED (e.g., an LED with a dominant wavelengthbetween the wavelength of blue and green) can be combined with a yellowLED to produce ‘green’ light. In general, the number of LEDs and thecolor of each LED can be selected as desired. Additional microdisplayscan also be included.

In some embodiments, the duty cycle for the lesser efficient LED (e.g.green) can be increased by various data compression techniques andalgorithms. For example, sending only the difference in imageinformation from the previous image rather than the total informationrequired to reconstruct each image allows an increase in the data rate.Using this method, less data needs to be sent allowing for higher datarates and reduced duty cycles for complementary colors for a givenrefresh cycle.

In embodiments in which multiple LEDs are used to illuminate a givenmicrodisplay, optical componentry may or may not be present along thelight path between one or more of the LEDs and the microdisplay. Forexample, an x-cube or a set of dichroic mirrors may be used to combinelight from the multiple LEDs onto a single microdisplay. In embodimentsin which optical componentry is present along the light path, differentoptical componentry can be used for each LED (e.g. if the surface of theLEDs are of different size or shape), or the same optical componentrycan be used for more than one LED.

In some embodiments, differing brightness for a particular color basedon the desired chromaticity of an image may be obtained by illuminatingthe display for a portion of the activation time allocated to theparticular LED. For example, to obtain an intense blue, the blue LED canbe activated for the entire activation time and for a less intense blue,the blue LED is activated for only a portion of the total allocatedactivation time. The portion of the activation time used to illuminatethe display can be modulated, for example, by a set of mirrors that canbe positioned to either pass light to the microdisplay or reflect thelight away from the microdisplay.

In certain embodiments, an array of moveable microdisplays (e.g., amoveable mirror) is actuated to produce a desired intensity. Forexample, each micromirror can represent a pixel and the intensity of thepixel can be determined by the positioning of the microdisplay. Forexample, the micromirror can be in an on or an off state and theproportion of the time spent in the on state during the activation timeof a particular color of LED determines the intensity of the image.

In general, in embodiments in which multiple LEDs are used, one or moreof the LEDs (e.g., each LED) can have the aspect ratio relationshipdescribed above with respect to the aspect ratio of microdisplay 1130.

FIG. 11 shows an optical display system 1600 that includes LED 1110,microdisplay 1130, a cooling system 1510, and a sensor 1520 that is inthermal communication with LED 1110 and electrical communication withcooling system 1510 so that, during use of system 1600, sensor 1520 andcooling system 1510 can be used to regulate the temperature of LED 1110.This can be desirable, for example, when LED 1110 is a relatively largeare LED (see discussion below) because such an LED can generate asignificant amount of heat. With the arrangement shown in FIG. 11, theamount of power input to LED 1110 can be increased with (primarily,increased operational efficiency at higher drive currents) reduced riskof damaging LED 1110 via the use of sensor 1520 and cooling system 1510to cool LED 1110. Examples of cooling systems include thermal electriccoolers, fans, heat pipes, and liquid cooling systems. Sensor 1520 canbe, for example, manually controlled or computer controlled. In someembodiments, the system may not include a sensor (e.g., cooling system1510 can be permanently on, or can be manually controlled). The use of acooling system can provide multiple advantages such as reducing thelikelihood of damage to the LED resulting from an excess temperature andincreasing the efficiency of the LED at higher drive currents. Thecooling system may also reduce the shift in wavelength induced bytemperature.

In some embodiments, using a non-lambertian LED results in non-uniformangular distribution of light. In such embodiments, the microdisplay canbe translated away from the image plane to reduce the appearance of theangular non-uniformity. In certain embodiments, information flow to themicrodisplay can be achieved using an electrical or optical connection.In some examples, the rate of information flow can be increased using anoptical connection.

In some embodiments, the size of a PLLED or other non-lambertian sourcecan be increased and the light can be collected at a smaller angle. Thiscan increase the brightness of the image on a display.

FIG. 12 shows a side view of an LED 100 in the form of a packaged die.LED 100 includes a multi-layer stack 122 disposed on a submount 120.Multi-layer stack 122 includes a 320 nm thick silicon doped (n-doped)GaN layer 134 having a pattern of openings 150 in its upper surface 110.Multi-layer stack 122 also includes a bonding layer 124, a 100 nm thicksilver layer 126, a 40 nm thick magnesium doped (p-doped) GaN layer 128,a 120 nm thick light-generating region 130 formed of multiple InGaN/GaNquantum wells, and a AlGaN layer 132. An n-side contact pad 136 isdisposed on layer 134, and a p-side contact pad 138 is disposed on layer126. An encapsulant material (epoxy having an index of refraction of1.5) 144 is present between layer 134 and a cover slip 140 and supports142. Layer 144 does not extend into openings 150.

Light is generated by LED 100 as follows. P-side contact pad 138 is heldat a positive potential relative to n-side contact pad 136, which causeselectrical current to be injected into LED 100. As the electricalcurrent passes through light-generating region 130, electrons fromn-doped layer 134 combine in region 130 with holes from p-doped layer128, which causes region 130 to generate light. Light-generating region130 contains a multitude of point dipole radiation sources that emitlight (e.g., isotropically) within the region 130 with a spectrum ofwavelengths characteristic of the material from which light-generatingregion 130 is formed. For InGaN/GaN quantum wells, the spectrum ofwavelengths of light generated by region 130 can have a peak wavelengthof about 445 nanometers (nm) and a full width at half maximum (FWHM) ofabout 30 nm.

It is to be noted that the charge carriers in p-doped layer 126 haverelatively low mobility compared to the charge carriers in the n-dopedsemiconductor layer 134. As a result, placing silver layer 126 (which isconductive) along the surface of p-doped layer 128 can enhance theuniformity of charge injection from contact pad 138 into p-doped layer128 and light-generating region 130. This can also reduce the electricalresistance of device 100 and/or increase the injection efficiency ofdevice 100. Because of the relatively high charge carrier mobility ofthe n-doped layer 134, electrons can spread relatively quickly fromn-side contact pad 136 throughout layers 132 and 134, so that thecurrent density within the light-generating region 130 is substantiallyuniform across the region 130. It is also to be noted that silver layer126 has relatively high thermal conductivity, allowing layer 126 to actas a heat sink for LED 100 (to transfer heat vertically from themulti-layer stack 122 to submount 120).

At least some of the light that is generated by region 130 is directedtoward silver layer 126. This light can be reflected by layer 126 andemerge from LED 100 via surface 110, or can be reflected by layer 126and then absorbed within the semiconductor material in LED 100 toproduce an electron-hole pair that can combine in region 130, causingregion 130 to generate light. Similarly, at least some of the light thatis generated by region 130 is directed toward pad 136. The underside ofpad 136 is formed of a material (e.g., a Ti/Al/Ni/Au alloy) that canreflect at least some of the light generated by light-generating region130. Accordingly, the light that is directed to pad 136 can be reflectedby pad 136 and subsequently emerge from LED 100 via surface 110 (e.g.,by being reflected from silver layer 126), or the light that is directedto pad 136 can be reflected by pad 136 and then absorbed within thesemiconductor material in LED 100 to produce an electron-hole pair thatcan combine in region 130, causing region 130 to generate light (e.g.,with or without being reflected by silver layer 126).

As shown in FIGS. 12 and 13, surface 110 of LED 100 is not flat butconsists of a modified triangular pattern of openings 150. In general,various values can be selected for the depth of openings 150, thediameter of openings 150 and the spacing between nearest neighbors inopenings 150 can vary. Unless otherwise noted, for purposes of thefigures below showing the results of numerical calculations, openings150 have a depth 146 equal to about 280 nm, a non-zero diameter of about160 nm, a spacing between nearest neighbors or about 220 nm, and anindex of refraction equal to 1.0. The triangular pattern is detuned sothat the nearest neighbors in pattern 150 have a center-to-centerdistance with a value between (a−Δa) and (a+Δa), where “a” is thelattice constant for an ideal triangular pattern and “Δa” is a detuningparameter with dimensions of length and where the detuning can occur inrandom directions. To enhance light extraction from LED 100 (seediscussion below), detuning parameter, Δa, is generally at least aboutone percent (e.g., at least about two percent, at least about threepercent, at least about four percent, at least about five percent) ofideal lattice constant, a, and/or at most about 25% (e.g., at most about20%, at most about 15%, at most about 10%) of ideal lattice constant, a.In some embodiments, the nearest neighbor spacings vary substantiallyrandomly between (a−Δa) and (a+Δa), such that pattern 150 issubstantially randomly detuned.

For the modified triangular pattern of openings 150, it has been foundthat a non-zero detuning parameter enhances the extraction efficiency ofan LED 100. For LED 100 described above, as the detuning parameter Δaincreases from zero to about 0.15a, numerical modeling (described below)of the electromagnetic fields in the LED 100 has shown that theextraction efficiency of the device increases from about 0.60 to about0.70, as shown in FIG. 14.

The extraction efficiency data shown in FIG. 14 are calculated by usinga three-dimensional finite-difference time-domain (FDTD) method toapproximate solutions to Maxwell's equations for the light within andoutside of LED 100. See, for example, K. S. Kunz and R. J. Luebbers, TheFinite-Difference Time-Domain Methods (CRC, Boca Raton, Fla., 1993); A.Taflove, Computational Electrodynamics: The Finite-DifferenceTime-Domain Method (Artech House, London, 1995), both of which arehereby incorporated by reference. To represent the optical behavior ofLED 100 with a particular pattern 150, input parameters in a FDTDcalculation include the center frequency and bandwidth of the lightemitted by the point dipole radiation sources in light-generating region130, the dimensions and dielectric properties of the layers withinmultilayer stack 122, and the diameters, depths, and nearest neighbordistances (NND) between openings in pattern 150.

In certain embodiments, extraction efficiency data for LED 100 arecalculated using an FDTD method as follows. The FDTD method is used tosolve the full-vector time-dependent Maxwell's equations:

${{\overset{\rightarrow}{\nabla}{\times \overset{\rightarrow}{E}}} = {{- \mu}\frac{\partial\overset{\rightarrow}{H}}{\partial t}}},\mspace{14mu}{{\overset{\rightarrow}{\nabla}{\times \overset{\rightarrow}{H}}} = {{ɛ_{\infty}\frac{\partial\overset{\rightarrow}{E}}{\partial t}} + \frac{\partial\overset{\rightarrow}{P}}{\partial t}}},$where the polarizability {right arrow over (P)}={right arrow over(P)}₁+{right arrow over (P)}₂+ . . . +{right arrow over (P)}_(m)captures the frequency-dependent response of the quantum welllight-generating region 130, the p-contact layer 126 and other layerswithin LED 100. The individual {right arrow over (P)}_(m) terms areempirically derived values of different contributions to the overallpolarizability of a material (e.g., the polarization response for boundelectron oscillations, the polarization response for free electronoscillations). In particular,

${{\frac{\mathbb{d}^{2}{\overset{\rightarrow}{P}}_{m}}{\mathbb{d}t^{2}} + {\gamma_{m}\frac{\mathbb{d}{\overset{\rightarrow}{P}}_{m}}{\mathbb{d}t}} + {\omega_{m}^{2}{\overset{\rightarrow}{P}}_{m}}} = {{ɛ(\omega)}\overset{\rightarrow}{E}}},$where the polarization corresponds to a dielectric constant

${ɛ(\omega)} = {ɛ_{\infty} + {\sum\limits_{m}{\frac{s_{m}}{\omega_{m}^{2} - \omega^{2} - {{\mathbb{i}}\;\gamma_{m}\omega}}.}}}$

For purposes of the numerical calculations, the only layers that areconsidered are encapsulant 144, silver layer 126 and layers betweenencapsulant 144 and silver layer 126. This approximation is based on theassumption that encapsulant 144 and layer 126 are thick enough so thatsurrounding layers do not influence the optical performance of LED 100.The relevant structures within LED 100 that are assumed to have afrequency dependent dielectric constant are silver layer 126 andlight-generating region 130. The other relevant layers within LED 100are assumed to not have frequency dependent dielectric constants. It isto be noted that in embodiments in which LED 100 includes additionalmetal layers between encapsulant 144 and silver layer 126, each of theadditional metal layers will have a corresponding frequency dependentdielectric constant. It is also to be noted that silver layer 126 (andany other metal layer in LED 100) has a frequency dependent term forboth bound electrons and free electrons, whereas light-generating region130 has a frequency dependent term for bound electrons but does not havea frequency dependent term for free electrons. In certain embodiments,other terms can be included when modeling the frequency dependence ofthe dielectric constant. Such terms may include, for example,electron-phonon interactions, atomic polarizations, ionic polarizationsand/or molecular polarizations.

The emission of light from the quantum well region of light-generatingregion 130 is modeled by incorporating a number of randomly-placed,constant-current dipole sources within the light-generating region 130,each emitting short Gaussian pulses of spectral width equal to that ofthe actual quantum well, each with random initial phase and start-time.

To cope with the pattern of openings 150 in surface 110 of the LED 100,a large supercell in the lateral direction is used, along with periodicboundary conditions. This can assist in simulating relatively large(e.g., greater than 0.01 mm on edge) device sizes. The full evolutionequations are solved in time, long after all dipole sources have emittedtheir energy, until no energy remains in the system. During thesimulation, the total energy emitted, the energy flux extracted throughtop surface 110, and the energy absorbed by the quantum wells and then-doped layer is monitored. Through Fourier transforms both in time andspace, frequency and angle resolved data of the extracted flux areobtained, and therefore an angle- and frequency-resolved extractionefficiency can be calculated. By matching the total energy emitted withthe experimentally known luminescence of light-generating region 130,absolute angle-resolved extraction in lumens/per solid angle/per chiparea for given electrical input is obtained.

Without wishing to be bound by theory, it is believed that the detunedpattern 150 can enhance the efficiency with which light generated inregion 130 emerges from LED 100 via surface 110 because openings 150create a dielectric function that varies spatially in layer 134according to pattern 150. It is believed that this alters the density ofradiation modes (i.e., light modes that emerge from surface 110) andguided modes (i.e., light modes that are confined within multi-layerstack 122) within LED 100, and that this alteration to the density ofradiation modes and guided modes within LED 100 results in some lightthat would otherwise be emitted into guided modes in the absence ofpattern 150 being scattered (e.g., Bragg scattered) into modes that canleak into radiation modes. In certain embodiments, it is believed thatpattern 150 (e.g., the pattern discussed above, or one of the patternsdiscussed below) can eliminate all of the guided modes within LED 100.

It is believed that the effect of detuning of the lattice can beunderstood by considering Bragg scattering off of a crystal having pointscattering sites. For a perfect lattice arranged in lattice planesseparated by a distance d, monochromatic light of wavelength λ isscattered through an angle θ according to the Bragg condition,nλ=2dsinθ, where n is an integer that gives the order of the scattering.However, it is believed that for a light source having a spectralbandwidth Δλ/λ and emitting into a solid angle ΔΘ, the Bragg conditioncan be relaxed by detuning the spacing of between lattice sites by adetuning parameter Δa. It is believed that detuning the latticeincreases the scattering effectiveness and angular acceptance of thepattern over the spectral bandwidth and spatial emission profile of thesource.

While a modified triangular pattern 150 having a non-zero detuningparameter Δa has been described that can enhance light extraction fromLED 100, other patterns can also be used to enhance light extractionfrom LED 100. When determining whether a given pattern enhances lightextraction from LED 100 and/or what pattern of openings may be used toenhance light extraction from LED 100, physical insight may first beused to approximate a basic pattern that can enhance light extractionbefore conducting such numerical calculations.

The extraction efficiency of LED 100 can be further understood (e.g., inthe weak scattering regime) by considering the Fourier transform of thedielectric function that varies spatially according to pattern 150. FIG.15 depicts the Fourier transform for an ideal triangular lattice.Extraction of light into a particular direction with in-plane wavevectork is related to the source emission S_(k′) into all those modes within-plane wavevector k′ (i.e. parallel to pattern 150) that arecompatible to k by the addition or subtraction of a reciprocal latticevector G, i.e k=k′±G. The extraction efficiency is proportional to themagnitude of the corresponding Fourier component (F_(k)) of thedielectric function ∈_(G) given by

${F_{\overset{\rightarrow}{k}} = {c_{\overset{\rightarrow}{k}}{\sum\limits_{\overset{\rightarrow}{G}}{ɛ_{\overset{\rightarrow}{G}}S_{\overset{\rightarrow}{k} - \overset{\rightarrow}{G}}}}}},\mspace{14mu}{ɛ_{\overset{\rightarrow}{G}} = {\int{{ɛ\left( \overset{\rightarrow}{r} \right)}{\mathbb{e}}^{{- {\mathbb{i}}}\;\overset{\rightarrow}{G}\overset{\rightarrow}{r}}{\mathbb{d}\overset{\rightarrow}{r}}}}}$

Since light propagating in the material generally satisfies the equationk²(in-plane)+k²(normal)=∈(ω/c)², the maximum G to be considered is fixedby the frequency (ω) emitted by the light-generating region and thedielectric constant of the light-generating region. As shown in FIG. 15,this defines a ring in reciprocal space which is often called the lightline. The light line will be an annulus due to the finite bandwidth ofthe light-generating region but for sake of clarity we illustrate thelight line of a monochromatic source. Similarly, light propagatingwithin the encapsulant is bounded by a light line (the inner circle inFIG. 15). Therefore, the extraction efficiency is improved by increasingF_(k) for all directions k that lie within the encapsulant light-linewhich amounts to increasing the number of G points within theencapsulant light line and increasing the scattering strength ∈_(G) forG points which lie within the material light line. This physical insightcan be used when selecting patterns that can improve extractionefficiency.

As an example, FIG. 16 shows the effect of increasing lattice constantfor an ideal triangular pattern. The data shown in FIG. 16 arecalculated using the parameters given for LED 100 shown in FIG. 12,except that the emitted light has a peak wavelength of 450 nm, and thedepth of the holes, the diameter of the holes, and the thickness of then-doped layer 134 scale with the nearest neighbor distance, a, as 1.27a,0.72a, and 1.27a+40 nm, respectively. Increasing the lattice constant,increases the density of G points within the light-line of theencapsulant. A clear trend in extraction efficiency with NND isobserved. It is believed that the maximum extraction efficiency occursfor NND approximately equal to the wavelength of light in vacuum. Thereason a maximum is achieved, is that as the NND becomes much largerthan the wavelength of light, the scattering effect is reduced becausethe material becomes more uniform.

As another example, FIG. 17 shows the effect of increasing hole size orfilling factor. The filling factor for a triangular pattern is given by(2π/√3)*(r/a)², where r is the radius of a hole. The data shown in FIG.17 are calculated using the parameters given for the LED 100 shown inFIG. 12, except that the diameter of the openings is changed accordingthe filling factor value given on the x-axis of the graph. Theextraction efficiency increases with filling factor as the scatteringstrengths (∈_(G)) increase. A maximum is observed for this particularsystem at a filling factor of ˜48%. In certain embodiments, LED 100 hasa filling factor of at least about 10% (e.g., at least about 15%, atleast about 20%) and/or at most about 90% (e.g., at most about 80%, atmost about 70%, at most about 60%).

While a modified triangular pattern has been described in which adetuning parameter relates to positioning of openings in the patternfrom the positions in an ideal triangular lattice, a modified (detuned)triangular pattern may also be achieved by modifying the holes in anideal triangular pattern while keeping the centers at the positions foran ideal triangular pattern. FIG. 18 shows an embodiment of such apattern. The enhancement in light extraction, the methodology forconducting the corresponding numerical calculation, and the physicalexplanation of the enhanced light extraction for a light-emitting devicehaving the pattern shown in FIG. 18 is generally the same as describedabove. In some embodiments, a modified (detuned) pattern can haveopenings that are displaced from the ideal locations and openings at theideal locations but with varying diameters.

In other embodiments, enhanced light extraction from a light-emittingdevice can be achieved by using different types of patterns, including,for example, complex periodic patterns and nonperiodic patterns. Asreferred to herein, a complex periodic pattern is a pattern that hasmore than one feature in each unit cell that repeats in a periodicfashion. Examples of complex periodic patterns include honeycombpatterns, honeycomb base patterns, (2×2) base patterns, ring patterns,and Archimidean patterns. As discussed below, in some embodiments, acomplex periodic pattern can have certain openings with one diameter andother openings with a smaller diameter. As referred to herein, anonperiodic pattern is a pattern that has no translational symmetry overa unit cell that has a length that is at least 50 times the peakwavelength of light generated by region 130. Examples of nonperiodicpatterns include aperiodic patterns, quasicrystalline patterns, Robinsonpatterns, and Amman patterns.

FIG. 19 shows numerical calculations for LED 100 for two differentcomplex periodic patterns in which certain openings in the patterns havea particular diameter, and other openings in the patterns have smallerdiameters. The numerical calculations represented in FIG. 19 show thebehavior of the extraction efficiency (larger holes with a diameter of80 nm) as the diameter of the smaller holes (dR) is varied from zero nmto 95 nm. The data shown in FIG. 17 are calculated using the parametersgiven for the LED 100 shown in FIG. 12 except that the diameter of theopenings is changed according the filling factor value given on thex-axis of the graph. Without wishing to be bound by theory, multiplehole sizes allow scattering from multiple periodicities within thepattern, therefore increasing the angular acceptance and spectraleffectiveness of the pattern. The enhancement in light extraction, themethodology for conducting the corresponding numerical calculation, andthe physical explanation of the enhanced light extraction for alight-emitting device having the pattern shown in FIG. 19 is generallythe same as described above.

FIG. 20 shows numerical calculations for LED 100 having different ringpatterns (complex periodic patterns). The number of holes in the firstring surrounding the central hole is different (six, eight or 10) forthe different ring patterns. The data shown in FIG. 20 are calculatedusing the parameters given for the LED 100 shown in FIG. 12, except thatthe emitted light has a peak wavelength of 450 nm. The numericalcalculations represented in FIG. 20 show the extraction efficiency ofLED 100 as the number of ring patterns per unit cell that is repeatedacross a unit cell is varied from two to four. The enhancement in lightextraction, the methodology for conducting the corresponding numericalcalculation, and the physical explanation of the enhanced lightextraction for a light-emitting device having the pattern shown in FIG.20 is generally the same as described above.

FIG. 21 shows numerical calculations for LED 100 having an Archimideanpattern. The Archimedean pattern A7 consists of hexagonal unit cells 230of 7 equally-spaced holes with a nearest neighbor distance of a. Withina unit cell 230, six holes are arranged in the shape of a regularhexagon and the seventh hole is located at the center of the hexagon.The hexagonal unit cells 230 then fit together along their edges with acenter-to-center spacing between the unit cells of a′=a*(1+√{square rootover (3)}) to pattern the entire surface of the LED. This is known as anA7 tiling, because 7 holes make up the unit cell. Similarly, theArchimidean tiling A19 consists of 19 equally-spaced holes with a NND ofa. The holes are arranged in the form of an inner hexagon of sevenholes, and outer hexagon of 12 holes, and a central hole within theinner hexagon. The hexagonal unit cells 230 then fit together alongtheir edges with a center-to-center spacing between the unit cells ofa′=a*(3+√{square root over (3)}) to pattern the entire surface of theLED. The enhancement in light extraction, the methodology for conductingthe corresponding numerical calculation, and the physical explanation ofthe enhanced light extraction for a light-emitting device having thepattern shown in FIG. 21 is generally the same as described above. Asshown in FIG. 21 the extraction efficiency for A7 and A19 is about 77%.The data shown in FIG. 21 are calculated using the parameters given forthe LED 100 shown in FIG. 12, except that the emitted light has a peakwavelength of 450 and except that the NND is defined as the distancebetween openings within an individual cell.

FIG. 22 shows numerical calculation data for LED 100 having aquasicrystalline pattern. Quasicrystalline patterns are described, forexample, in M. Senechal, Quasicrystals and Geometry (CambridgeUniversity Press, Cambridge, England 1996), which is hereby incorporatedby reference. The numerical calculations show the behavior of theextraction efficiency as the class of 8-fold based quasi-periodicstructure is varied. It is believed that quasicrystalline patternsexhibit high extraction efficiency due to high degree of in-planerotational symmetries allowed by such structures. The enhancement inlight extraction, the methodology for conducting the correspondingnumerical calculation, and the physical explanation of the enhancedlight extraction for a light-emitting device having the pattern shown inFIG. 22 is generally the same as described above. Results from FDTDcalculations shown in FIG. 22 indicate that the extraction efficiency ofquasicrystalline structures reaches about 82%. The data shown in FIG. 22are calculated using the parameters given for the LED 100 shown in FIG.12, except that the emitted light has a peak wavelength of 450 andexcept that the NND is defined as the distance between openings withinan individual cell.

While certain examples of patterns have been described herein, it isbelieved that other patterns can also enhance the light extraction fromLED 100 if the patterns satisfy the basic principles discussed above.For example, it is believed that adding detuning to quasicrystalline orcomplex periodic structures can increase extraction efficiency.

In some embodiments, at least about 45% (e.g., at least about 50%, atleast about 55%, at least about 60%, at least about 70%, at least about80%, at least about 90%, at least about 95%) of the total amount oflight generated by light-generating region 130 that emerges from LED 100emerges via surface 110.

In certain embodiments, the cross-sectional area of LED 100 can berelatively large, while still exhibiting efficient light extraction fromLED 100. For example, one or more edges of LED 100 can be at least aboutone millimeter (e.g., at least about 1.5 millimeters, at least about twomillimeters, at least about 2.5 millimeters, at least about threemillimeters), and at least about 45% (e.g., at least about 50%, at leastabout 55%, at least about 60%, at least about 70%, at least about 80%,at least about 90%, at least about 95%) of the total amount of lightgenerated by light-generating region 130 that emerges from LED 100emerges via surface 10. This can allow for an LED to have a relativelylarge cross-section (e.g., at least about one millimeter by at leastabout one millimeter) while exhibiting good power conversion efficiency.

In some embodiments, the extraction efficiency of an LED having thedesign of LED 100 is substantially independent of the length of the edgeof the LED. For example, the difference between the extractionefficiency of an LED having the design of LED 100 and one or more edgeshaving a length of about 0.25 millimeter and the extraction efficiencyof LED having the design of LED 100 and one or more edges having alength of one millimeter can vary by less than about 10% (e.g., lessthan about 8%, less than about 5%, less than about 3%). As referred toherein, the extraction efficiency of an LED is the ratio of the lightemitted by the LED to the amount of light generated by the device (whichcan be measured in terms of energy or photons). This can allow for anLED to have a relatively large cross-section (e.g., at least about onemillimeter by at least about one millimeter) while exhibiting good powerconversion efficiency.

In certain embodiments, the quantum efficiency of an LED having thedesign of LED 100 is substantially independent of the length of the edgeof the LED. For example, the difference between the quantum efficiencyof an LED having the design of LED 100 and one or more edges having alength of about 0.25 millimeter and the quantum efficiency of LED havingthe design of LED 100 and one or more edges having a length of onemillimeter can vary by less than about 10% (e.g., less than about 8%,less than about 5%, less than about 3%). As referred to herein, thequantum efficiency of an LED is the ratio of the number of photonsgenerated by the LED to the number of electron-hole recombinations thatoccur in the LED. This can allow for an LED to have a relatively largecross-section (e.g., at least about one millimeter by at least about onemillimeter) while exhibiting good performance.

In some embodiments, the wall plug efficiency of an LED having thedesign of LED 100 is substantially independent of the length of the edgeof the LED. For example, the difference between the wall plug efficiencyof an LED having the design of LED 100 and one or more edges having alength of about 0.25 millimeter and the wall plug efficiency of LEDhaving the design of LED 100 and one or more edges having a length ofone millimeter can vary by less than about 10% (e.g., less than about8%, less than about 5%, less than about 3%). As referred to herein, thewall plug efficiency of an LED is the product of the injectionefficiency of the LED (the ratio of the numbers of carriers injectedinto the device to the number of carriers that recombine in thelight-generating region of the device), the radiative efficiency of theLED (the ratio of electron-hole recombinations that result in aradiative event to the total number of electron-hole recombinations),and the extraction efficiency of the LED (the ratio of photons that areextracted from the LED to the total number of photons created). This canallow for an LED to have a relatively large cross-section (e.g., atleast about one millimeter by at least about one millimeter) whileexhibiting good performance.

In some embodiments, it may be desirable to manipulate the angulardistribution of light that emerges from LED 100 via surface 10. Toincrease extraction efficiency into a given solid angle (e.g., into asolid angle around the direction normal to surface 10) we examine theFourier transform of the dielectric function that varies spatiallyaccording to pattern 150 (as described earlier). FIG. 23 shows theFourier transform construction for two ideal triangular lattices ofdifferent lattice constant. To increase the extraction efficiency, weseek to increase the number of G points within the encapsulant lightline and scattering strengths of G points (∈_(G)) within the materiallight line. This would imply increasing the NND so as to achieve theeffect depicted in FIG. 16. However, here we are concerned withincreasing the extraction efficiency into a solid angle centered aroundthe normal direction. Therefore, we would also like to limit theintroduction of higher order G points by reducing the radius of theencapsulant light line, such that the magnitude of G>(ω(n_(e)))/c. Wecan see that by decreasing the index of refraction of the encapsulant(the bare minimum of which is removing the encapsulant all together) weallow larger NND and therefore increase the number of G points withinthe material light line that are available to contribute to extractionin the normal direction (F_(k=0)) while simultaneously avoidingdiffraction into higher order (oblique angles) in the encapsulant. Theabove described trends are depicted in FIG. 24 which shows extractionefficiency into a solid angle (given by the collection half-angle in thediagram). The data shown in FIG. 24 are calculated using the parametersgiven for the LED 100 shown in FIG. 12, except that the emitted lighthas a peak wavelength of 530 nm and a bandwidth of 34 nm, the index ofrefraction of the encapsulant was 1.0, the thickness of the p-dopedlayer was 160 nm, the light generating layer was 30 nm thick, the NND(a) for the three curves is shown on FIG. 24, and the depth, holediameter, and n-doped layer thickness scaled with a, as 1.27a, 0.72a,and 1.27a+40 nm, respectively. As the lattice constant is increased, theextraction efficiency at narrow angles increases as well as the overallextraction efficiency into all angles. However, for even larger latticeconstant, diffraction into higher order modes in the encapsulant limitsthe extraction efficiency at narrow angles even though the overallextraction efficiency increases into all angles. For a lattice constantof 460 nm, we calculate greater than 25% extraction efficiency into acollection half-angle of 30°. That is, about half of the extracted lightis collected within only about 13.4% of the upper hemisphere of solidangle demonstrating the collimation effect of the pattern. It isbelieved that any pattern that increases the number of G points withinthe material light line while limiting the number of G points within theencapsulant light line to only the G points at k=0 can improve theextraction efficiency into a solid angle centered around the normaldirection.

The approach is especially applicable for reducing the source etenduewhich is believed to often be proportional to n², where n is the indexof refraction of the surrounding material (e.g., the encapsulant). It istherefore believed that reducing the index of refraction of theencapsulating layer for LED 100 can lead to more collimated emission, alower source etendue, and therefore to a higher surface brightness (heredefined as the total lumens extracted into the etendue of the source).In some embodiments then, using an encapsulant of air will reduce thesource etendue while increasing extraction efficiency into a givencollection angle centered around the normal direction.

In certain embodiments, when light generated by region 130 emerges fromLED 100 via surface 110, the distribution of light is more collimatedthan a lambertian distribution. For example, in some embodiments, whenlight generated by region 130 emerges from LED 100 via surface 110, atleast about 40% (e.g., at least about 50%, at least about 70%, at leastabout 90%) of the light emerging via the surface of the dielectric layeremerges within at most about 30° (e.g., at most about 25°, at most about20°, at most about 15°) of an angle normal to surface 110.

The ability to extract a relatively high percentage of light from adesired angle alone or coupled with a relatively high light extractioncan allow for a relatively high density of LEDs to be prepared on agiven wafer. For example, in some embodiments, a wafer has at leastabout five LEDs (e.g., at least about 25 LEDs, at least about 50 LEDs)per square centimeter.

In some embodiments, it may be desirable to modify the wavelength(s) oflight that emerge(s) from a packaged LED 100 relative to thewavelength(s) of light generated by light-generating region 130. Forexample, as shown in FIG. 25, an LED 300 having a layer containing aphosphor material 180 can be disposed on surface 110. The phosphormaterial can interact with light at the wavelength(s) generated byregion 130 to provide light at desired wavelength(s). In someembodiments, it may be desirable for the light that emerges frompackaged LED 100 to be substantially white light. In such embodiments,the phosphor material in layer 180 can be formed of, for example, a(Y,Gd)(Al,Ga)G:Ce³⁺ or “YAG” (yttrium, aluminum, garnet) phosphor. Whenpumped by blue light emitted from the light-generating region 130, thephosphor material in layer 180 can be activated and emit light (e.g.,isotropically) with a broad spectrum centered around yellow wavelengths.A viewer of the total light spectrum emerging from packaged LED 100 seesthe yellow phosphor broad emission spectrum and the blue InGaN narrowemission spectrum and typically mixes the two spectra to perceive white.

In certain embodiments, layer 180 can be substantially uniformlydisposed on surface 110. For example, the distance between the top 151of pattern 150 and the top 181 of layer 180 can vary by less than about20% (e.g., less than about 10%, less than about 5%, less than about 2%)across surface 110.

In general, the thickness of layer 180 is small compared to thecross-sectional dimensions of surface 130 of LED 100, which aretypically about one millimeter (mm) by one mm. Because layer 180 issubstantially uniformly deposited on surface 110, the phosphor materialin layer 180 can be substantially uniformly pumped by light emerging viasurface 110. The phosphor layer 180 is relatively thin compared to thedimensions of the surface 110 of the LED 100, such that light emitted bythe light-generating region 130 is converted into lower wavelength lightwithin the phosphor layer 180 approximately uniformly over the entiresurface 110 of LED 100. Thus, the relatively thin, uniform phosphorlayer 180 produces a uniform spectrum of white light emitted from theLED 100 as a function of position on surface 110.

In general, LED 100 can be fabricated as desired. Typically, fabricationof LED 100 involves various deposition, laser processing, lithography,and etching steps.

For example, FIG. 26 shows a LED wafer 500 containing an LED layer stackof material deposited on a substrate (e.g., sapphire, compoundsemiconductor, zinc oxide, silicon carbide, silicon) 502. Such wafersare commercially available. Exemplary commercial suppliers includeEpistar Corporation, Arima Optoelectronics Corporation and South EpitaxyCorporation. On substrate 502 are disposed, consecutively, a bufferlayer 504 (e.g., a nitride-containing layer, such as a GaN layer, an AlNlayer, an AlGaN layer), an n-doped semiconductor layer (e.g., an n-dopedSi:GaN) layer 506, a current spreading layer 508 (e.g., an AlGaN/GaNheterojunction or superlattice), a light-emitting region 510 (e.g., anInGaN/GaN multi-quantum well region), and a semiconductor layer 512(e.g., a p-doped Mg:GaN layer). Wafer 500 generally has a diameter of atleast about two inches (e.g., from about two inches to about 12 inches,from about two inches to about six inches, from about two inches toabout four inches, from about two inches to about three inches).

FIG. 27 shows a multi-layer stack 550 including layers 502, 504, 506,508, 510 and 512, as well as layers 520, 522, 524 and 526, which aregenerally formed of materials capable of being pressure and/or heatbonded as described below. For example, layer 520 can be a nickel layer(e.g., electron-beam evaporated), layer 522 can be a silver layer (e.g.,electron-beam evaporated), layer 524 can be a nickel layer (e.g.,electron-beam evaporated), and layer 526 can be a gold layer (e.g.,electron-beam evaporated). In some embodiments, layer 520 can be arelatively thin layer, and layer 524 can be a relatively thick layer.Layer 524 can act, for example, as diffusion barrier to reduce thediffusion of contaminants (e.g., gold) into layers 520, 522 and/or 524itself. After deposition of layers 520, 522, 524 and 526, multi-layerstack 550 can be treated to achieve an ohmic contact. For example, stack550 can be annealed (e.g., at a temperature of from about 400° C. toabout 600° C.) for a period of time (e.g., from about 30 seconds toabout 300 seconds) in an appropriate gas environment (e.g., nitrogen,oxygen, air, forming gas).

FIG. 28 shows a multi-layer stack 600 that includes a submount (e.g.,germanium (such as polycrystalline germanium), silicon (such aspolycrystalline silicon), silicon-carbide, copper, copper-tungsten,diamond, nickel-cobalt) 602 having layers 604, 606, 608 and 610deposited thereon. Submount 602 can be formed, for example, bysputtering or electroforming. Layer 604 is a contact layer and can beformed, for example, from aluminum (e.g., electron evaporated). Layer606 is a diffusion barrier and can be formed, for example, from Ni (e.g.electron evaporated). Layer 608 can be a gold layer (e.g., electron-beamevaporated), and layer 610 can be a AuSn bonding layer (e.g., thermalevaporated, sputtered) onto layer 608. After deposition of layers 604,606, 608 and 610, multi-layer stack 600 can be treated to achieve anohmic contact. For example, stack 600 can be annealed (e.g., at atemperature of from about 350° C. to about 500° C.) for a period of time(e.g., from about 30 seconds to about 300 seconds) in an appropriate gasenvironment (e.g., nitrogen, oxygen, air, forming gas).

FIG. 29 shows a multi-layer stack 650 formed by bonding together layers526 and 610 (e.g., using a solder bond, using a eutectic bond, using aperitectic bond). Layers 526 and 610 can be bonded, for example, usingthermal-mechanical pressing. As an example, after contacting layers 526and 610, multi-layer stack 650 can be put in a press and pressurized(e.g., using a pressure of up to about 5 MPa, up to about 2 MPa) heated(e.g., to a temperature of from about 200° C. to about 400° C.). Stack650 can then be cooled (e.g., to room temperature) and removed from thepress.

Substrate 502 and buffer layer 504 are then at least partially removedfrom stack 650. In general, this can be achieved using any desiredmethods. For example, as shown in FIG. 30, in some embodiments,substrate 502 is removed by exposing stack 650 (e.g., through surface501 of substrate 502) to electromagnetic radiation at an appropriatewavelength to partially decompose layer 504. It is believed that thisresults in local heating of layer 504, resulting in the partialdecomposition of the material of layer 504 adjacent the interface oflayer 504 and substrate 502, thereby allowing for the removal ofsubstrate 502 from stack 650 (see discussion below). For example, inembodiments in which layer 504 is formed of gallium nitride, it isbelieved that constituents including gallium and gaseous nitrogen areformed. In some embodiments, stack 650 can be heated during exposure ofsurface 501 to the electromagnetic radiation (e.g., to reduce strainwithin stack 650). Stack 650 can be heated, for example, by placingstack 650 on a hot plate and/or by exposing stack 650 to an additionallaser source (e.g. a CO₂ laser). Heating stack 650 during exposure ofsurface 501 to electromagnetic radiation can, for example, reduce (e.g.,prevent) liquid gallium from re-solidifying. This can reduce the buildup of strain within stack 650 which can occur upon the re-solidificationof the gallium.

In certain embodiments, after exposure to the electromagnetic radiation,residual gallium is present and keeps substrate 502 bonded in stack 650.In such embodiments, stack 650 can be heated to above the meltingtemperature of gallium to allow substrate 502 to be removed from thestack. In certain embodiments, stack 650 may be exposed to an etchant(e.g., a chemical etchant, such as HCl) to etch the residual gallium andremove substrate 502. Other methods of removing the residual gallium(e.g., physical methods) may also be used.

As an example, in certain embodiments, surface 501 is exposed to laserradiation including the absorption wavelength of layer 504 (e.g., about248 nanometers, about 355 nanometers). Laser radiation processes aredisclosed, for example, in U.S. Pat. Nos. 6,420,242 and 6,071,795, whichare hereby incorporated by reference. The multi-layer stack is thenheated to above the melting point of gallium, at which point substrate502 and buffer layer 504 are removed from the stack by applying alateral force to substrate 502 (e.g., using a cotton swab).

In some embodiments, multiple portions of surface 501 are simultaneouslyexposed to the electromagnetic radiation. In certain embodiments,multiple portions of surface 501 are sequentially exposed toelectromagnetic radiation. Combinations of simultaneous and sequentialexposure can be used. Further, the electromagnetic radiation can beexposed on surface 501 in the form of a pattern (e.g., a serpentinepattern, a circular pattern, a spiral pattern, a grid, a grating, atriangular pattern, an elementary pattern, a random pattern, a complexpattern, a periodic pattern, a nonperiodic pattern). In someembodiments, the electromagnetic radiation can be rastered across one ormore portions of surface 501. In certain embodiments, surface 501 isexposed to overlapping fields of electromagnetic radiation.

In some embodiments, the electromagnetic radiation passes through a maskbefore reaching surface 501. As an example, the electromagneticradiation can pass through an optical system that includes a mask (e.g.,a high thermal conductivity mask, such as a molybdenum mask, acopper-beryllium mask) before reaching surface 501. In some embodiments,the mask is an aperture (e.g., for truncating or shaping the beam). Theoptical system can include, for example, at least two lenses having themask disposed therebetween. As another example, the mask can be formedas a pattern of material on surface 501, with the mask leaving certainportions of surface 501 exposed and some portions of surface 501unexposed. Such a mask can be formed, for example, via a lithographyprocess. In some embodiments, the electromagnetic radiation can berastered across one or more portions of the mask.

Without wishing to be bound by theory, it is believed that reducing atleast one dimension of the region on surface 501 exposed toelectromagnetic radiation within a given area of surface 501 can limitundesired crack propagation, such as crack propagation into layer 504,layer 506 or other layers of stack 650 during removal of substrate 502,while still allowing for crack propagation at the interface betweensubstrate 502 and buffer layer 504. It is believed that, if the size ofthe feature of the electromagnetic radiation on surface 501 is toolarge, then a gaseous bubble (e.g., a nitrogen bubble) may form that cancreate a localized pressure that can cause undesired cracking. Forexample, in embodiments in which surface 501 is exposed to laserradiation that forms a spot or a line on surface 501, at least onedimension of the spot or line can be a maximum of at most about onemillimeter (e.g., at most about 500 microns, at most about 100 microns,at most about 25 microns, at most about 10 microns). In someembodiments, the spot size is from about five microns to about onemillimeter (e.g., from about five microns to about 100 microns, fromabout five microns to about 25 microns, from about five microns to about10 microns).

In certain embodiments, stack 650 is vibrated while surface 501 isexposed to the electromagnetic radiation. Without wishing to be bound bytheory, it is believed that vibrating stack 650 while exposing stack 650to the electromagnetic radiation can enhance crack propagation along theinterface between layer 504 and substrate 502. Generally, the conditionsare selected to limit the propagation of cracks into layer 504 (e.g., sothat substantially no cracks propagate into layer 504, 506, and the restof stack 650).

After removal of substrate 502, a portion of buffer layer 504 typicallyremains on at least a portion of the surface of layer 506. A residue ofmaterial from substrate 502 (e.g., containing aluminum and/or oxygen)can also be present on the remaining portion of buffer layer 504 and/oron the surface of layer 506. It is generally desirable to remove theremaining portions of buffer layer 504 and any residue from substrate502, to expose the surface of layer 506, and to clean the exposedsurface of layer 506 because layer 506 (which is typically formed of ann-doped semiconductor material) can exhibit good electrical properties(e.g., desirable contact resistance) for subsequent formation of anelectrical contact. One or more process steps are usually used to removeany residue and/or remaining portion of buffer layer 504 present, and toclean the surface of layer 506 (e.g., to remove impurities, such asorganics and/or particles). The process(es) can be performed using avariety of techniques and/or combinations of techniques. Examplesinclude chemical-mechanical polishing, mechanical polishing, reactiveion etching (e.g., with a substantially chemically etching component),physical etching, and wet etching. Such methods are disclosed, forexample, in Ghandhi, S., VLSI Fabrication Principles: Silicon & GalliumArsenide (1994), which is hereby incorporated by reference. In certainembodiments, buffer layer 504 is not completely removed. Instead, insuch embodiments, these processes can be used to remove only on portionsof buffer layer 504 that correspond to locations where electrical leadswill subsequently be disposed (e.g., by using a self-aligned process).

Often, when substrate 502 is removed, the amount of strain in stack 650(e.g., due to the lattice mismatch and/or thermal mismatch between thelayers in stack 650) can change. For example, if the amount of strain instack 650 is decreased, the peak output wavelength of region 510 canchange (e.g., increase). As another example, if the amount of strain instack 650 is increased, the peak output wavelength of region 510 canchange (e.g., decrease).

To limit undesired cracking during removal of substrate 502, in someembodiments, consideration is given to the coefficient of thermalexpansion of both substrate 502, the coefficient of thermal expansion ofsubmount 602, the combined thickness of layers 504, 506, 508, 510, and512, and/or the coefficient of thermal expansion of one or more oflayers 504, 506, 508, 510, and 512. As an example, in some embodiments,substrate 502 and submount 602 are selected so that the coefficient ofthermal expansion of submount 602 differs from a coefficient of thermalexpansion of substrate 502 by less than about 15% (e.g., less than about10%, less than about 5%). As another example, in certain embodiments,substrate 502 and submount 602 are selected so that the thickness ofsubmount 602 is substantially greater than the thickness of substrate502. As an additional example, in some embodiments, semiconductor layers504, 506, 508, 510, 512 and submount 602 are selected so that thecoefficient of thermal expansion of submount 602 differs from acoefficient of thermal expansion of one or more of layers 504, 506, 608,510, and 512 by less than about 15% (e.g., less than about 10%, lessthan about 5%).

In general, substrate 502 and submount 602 can have any desiredthickness. In some embodiments, substrate 502 is at most about fivemillimeters (e.g., at most about three millimeters, at most about onemillimeter, about 0.5 millimeter) thick. In certain embodiments,submount 602 is at most about 10 millimeters (e.g., at most about fivemillimeters, at most about one millimeter, about 0.5 millimeter) thick.In some embodiments, submount 602 is thicker than substrate 502, and, incertain embodiments, substrate 502 is thicker than submount 602.

After removal of buffer layer 504 and exposing/cleaning the surface oflayer 506, the thickness of layer 506 can be reduced to a desired finalthickness for use in the light-emitting device. This can be achieved,for example, using a mechanical etching process, alone or in combinationwith an etching process. In some embodiments, after etching/cleaning theexposed surface of layer 506, the surface of layer 506 has a relativelyhigh degree of flatness (e.g., a relatively high degree of flatness onthe scale of the lithography reticle to be used). As an example, in someembodiments, after etching/cleaning the exposed surface of layer 506,the surface of layer 506 has a flatness of at most about 10 microns per6.25 square centimeters (e.g., at most about five microns per 6.25square centimeters, at most about one micron per 6.25 squarecentimeters). As another example, in certain embodiments, afteretching/cleaning the exposed surface of layer 506, the surface of layer506 has a flatness of at most about 10 microns per square centimeter(e.g., at most about five microns per square centimeter, at most aboutone microns per square centimeter). In certain embodiments, afteretching/cleaning the exposed surface of layer 506, the surface of layer506 has an RMS roughness of at most about 50 nanometers (e.g., at mostabout 25 nanometers, at most about 10 nanometers, at most about fivenanometers, at most about one nanometer).

In some embodiments, prior to forming the dielectric function thatvaries spatially according to a pattern in the surface of layer 506, theexposed surface of layer 506 may be too rough and/or insufficiently flatto use nanolithography to form the pattern with sufficient accuracyand/or reproducibility. To enhance the ability to accurately and/orreproducibly form the pattern in the surface of layer 506, thenanolithography process may include depositing a planarization layer onthe surface of layer 506 and a lithography layer on the surface of theplanarization layer. For example, FIG. 31 shows an embodiment in which aplanarization layer 702 is disposed on the surface of layer 506, and alithography layer 704 is disposed on the surface of layer 702, anexposed surface 505 of layer 506 may be relatively rough (e.g., RMSroughness of about 10 nanometers or more) after cleaning/etching layer506. In some embodiments, planarization layer 702 is formed of multiplelayers (e.g., of the same material) that are sequentially deposited.

Examples of materials from which planarization layer 702 can be selectedinclude polymers (e.g., DUV-30J from Brewer Sciences, anti-reflectioncoatings, high viscosity formable polymers), and examples of materialsfrom which lithography layer 704 can be selected include UV-curablepolymers (e.g., low viscosity MonoMat™ available from MolecularImprints, Inc.). Layers 702 and 704 can be formed using any desiredtechnique, such as, for example, spin coating, vapor deposition, and thelike.

Layer 702 can be, for example, at least about 100 nanometers thick(e.g., at least about 500 nanometers thick) and/or at most about fivemicrons thick (e.g., at most about one micron thick). Layer 704 can be,for example, at least about one nanometer thick (e.g., at least about 10nanometers thick) and/or at most about one micron thick (e.g., at mostabout 0.5 micron thick).

A mold that defines a portion of the desired pattern is then pressedinto lithography layer and (typically with heating or UV-curing of themold and/or layer 704), and stepped across the surface of layer 704 in aportion-by-portion manner to form indentions in layer 704 (FIG. 32) thatcorrespond to the desired pattern in the surface of layer 506. In someembodiments, a single step covers the entire wafer (e.g., full wafernanolithography techniques). Layer 704 is then etched (e.g., usingreactive ion etching, wet etching) to expose portions of the surface oflayer 702 corresponding to what were the indented portions of layer 704(FIG. 33). Examples of such imprint/etch processes are disclosed, forexample, in U.S. Pat. No. 5,722,905, and Zhang et al., Applied PhysicsLetters, Vol. 83, No. 8, pp. 1632-34, both of which are herebyincorporated by reference. Typically, the pattern in layer 704 alsoleaves regions for depositing n-contacts later on in the process flow.In alternate embodiments, other techniques (e.g., x-ray lithography,deep ultraviolet lithography, extreme ultraviolet lithography, immersionlithography, interference lithography, electron beam lithography,photolithography, microcontact printing, self-assembly techniques) maybe used to create the pattern in layer 704.

As shown in FIG. 34, patterned layer 704 is used as a mask to transferthe pattern into the planarization layer 702 (e.g., dry etching, wetetching). An example of a dry etching method is reactive ion etching.Referring to FIG. 35, layers 702 and 704 are subsequently used as a maskto transfer the pattern into the surface of layer 506 (e.g., using dryetching, wet etching). As shown in FIG. 36, following etching of layer506, the layers 702 and 704 are removed (e.g., using an oxygen-basedreactive ion etch, a wet solvent etch).

Referring to FIG. 37, in some embodiments, the process can include,disposing a material 708 (e.g., a metal, such as aluminum, nickel,titanium, tungsten) in the etched portions of layers 702 and 704 (e.g.,by evaporation) and on the surface of layer 704. As shown in FIG. 38,layers 702 and 704 are then etched (e.g., using reactive ion etching,wet etching), leaving behind etch-resistant material 708 on the surfaceof layer 506, which can serve as a mask for etching the pattern into thesurface of layer 506 (FIG. 39). Referring to FIG. 40, etch resistantmaterial 708 can then be removed (e.g., using dry etching, wet etching).

In some embodiments, the process can include, after forming the indentsin layer 704, disposing (e.g., spin coating) an etch resistant material(e.g., a Si-doped polymer) 710 on the surface of layer 704 and in theindents in layer 704, and material 710 is then etched back (e.g., usingdry etching) so that to expose the surface of layer 704 whilemaintaining the etch-resistant material in the indents in layer 704(FIG. 41). As shown in FIG. 42, portions of layers 702 and 704 are thenetched (e.g., using reactive ion etching, dry etching, wet etching),leaving behind etch-resistant material 708 and the portions of layers702 and 704 under material 708, which serve as a mask for etching thepattern into the surface of layer 506 (FIG. 43). Referring to FIG. 44,the remaining portions of layers 702 and 704, as well as etch resistantmaterial 708, can then be removed (e.g., using reactive ion etching, dryetching, wet etching). In some embodiments, removing layer 708 caninvolve the use of a plasma process (e.g., a fluorine plasma process).

After the pattern has been transferred to n-doped layer 506, a layer ofphosphor material can optionally be disposed (e.g., spin-coated) ontothe patterned surface of n-doped layer 506. In some embodiments, thephosphor can conformally coat the patterned surface (coat withsubstantially no voids present along the bottoms and sidewalls of theopenings in the patterned surface). Alternatively, a layer ofencapsulant material can be disposed on the surface of patterned n-dopedlayer 506 (e.g. by CVD, sputtering, suspension by liquid binder that issubsequently evaporated). In some embodiments, the encapsulant cancontain one or more phosphor materials. In some embodiments, thephosphor can be compressed to achieve thickness uniformity less thanabout 20%, less than about 15%, less than about 10%, less than about 5%,or less than about 2% of the average thickness of the phosphor. In someembodiments, the phosphor-containing encapsulant can conformally coatthe patterned surface.

After the dielectric function pattern has been created in the n-dopedlayer 506, individual LED dice can be cut from the wafer. Once waferprocessing and wafer testing is complete, individual LED dice areseparated and prepared for packaging and testing. A sidewall passivationstep and/or a pre-separation deep mesa etching step may be used toreduce potential damage to the electrical and/or optical properties ofthe patterned LED incurred during wafer cutting. The individual LEDs canbe any size up to the size of the wafer itself, but individual LEDs aretypically square or rectangular, with sides having a length betweenabout 0.5 mm to 5 mm. To create the dice, standard photolithography isused to define the location of contact pads on the wafer for energizingthe device, and ohmic contacts are evaporated (e.g. using electron beamevaporation) onto the desired locations.

In some embodiments, as shown in FIG. 45A, a contact layout for an LED1802 includes two conductive pads 1804 a and 1804 b and conductive bars(or fingers) 1806 extending from conductive pads 1804 a and 1804 btoward a central area of LED 1802. Wire bonds (not shown) connected toconductive pads 1804 a and 1804 b provide current and voltage to LED1802. Conductive bars 1806 spread the current from the conductive pads1804 a and 1804 b to a top surface 1808 of LED 1802. Bars 1806 allow thecurrent to be spread sufficiently across top surface 1808 while limitingthe amount of surface 1808 covered by the contacts.

FIG. 45B shows a top view of LED 1802 including conductive pads 1804 aand 1804 b and conductive bars 1806. In some embodiments, the width ofconductive pads 1804 a and 1804 b can be larger than the width ofconductive bars 1806. The larger width of pads 1804 a and 1804 b canallow pads 1804 a and 1804 b to function as power busses and spread arelatively large amount of power down the bus to bars 1806. The width ofpads 1804 a and 1804 b and bars 1806 can be relative to the size of LED1802 and/or can be based on other factors such as lithography andprocessing parameters.

For example, an LED may range in size from about 0.5 mm to about 1 cm ona side. As described above, the aspect ratio of LED 1802 can also vary.The width of conductive pads 1804 a and 1804 b can be, for example,about 50 um to about 500 um and the width of bars 1806 can be, forexample, about 1 um to about 50 um. The height of conductive pads 1804 aand 1804 b and bars 1806 can vary based on, for example, current andpower to be supplied to the LED or based on deposition and processingparameters. For example, conductive pads 1804 a and 1804 b and bars 1806can be about 0.1 um to about 10 um in height.

In general, bars 1806 can vary as desired in both length and shape. Asshown in FIG. 45B, bars 1806 can be rectangular and extend fromconductive pads 1804 a and 1804 b toward a central region of LED 1802.Alternatively, bars 1806 could have a different shape such as square,triangular, or trapezoidal.

FIGS. 46A to 46C show another example of a contact structure. In thisexample, multiple bars 1812 extend across the entire length of LED 1810,connecting conductive pad 1804 a to conductive pad 1804 b. Contact bars1812 have an associated resistivity r_(m), thickness t_(b), and a lengthl. Current distribution properties for LED 1810 based on conductive pads1804 a and 1804 b and contact bars 1812 can be estimated by simplifyingthe structure into an equivalent circuit model as shown in FIG. 46C.

The aspect ratio of LED 1810 can influence the current dissipation ofthe system. The aspect ratio ‘L’ of LED 1810 can be calculated accordingto the following equation as shown below:L=√{square root over (Ab/a)}

where A is the die's surface area (e.g., length multiplied by width) anda and b are the aspect ratios of the die. For example for an LED with a16×9 aspect ratio, a=16 and b=9.

As described above, in order to allow light generated in the LED to beemitted through the surface, contact bars 1812 do not cover the entiresurface of LED 1810. Since the contacts cover only a portion of thesurface of LED 1810, the contact resistance is divided by the surfacecoverage ratio f, as shown in the following equationρ_(n-c)→ρ_(n-c) /f

The current density across the junction can be estimated according tothe following equation as shown below:J=J ₀(e ^(eV) ^(j) ^(/KT)−1),

where J₀ is the junction saturation current and T the absolutetemperature. The above estimates neglect the contribution of the n-typematerial in lateral current spreading. However, in general the currentspreading is predominantly occurring in the metal contact because theconductivity of the contact is much greater than the conductivity of then-type material. For example, the ratio of the contact conductivity tothe n-type material conductivity can be in the range of from about 100to about 500.

In a similar system (but with infinite separation between the pads), ifthe calculation is performed in a forward bias (e.g., V_(j)>>kT/e) andif the voltage drop across the series resistance is much larger thankT/e (e.g., (ρ_(p-c)+ρ_(n-c)/f+ρ_(p)t_(p)+ρ_(n)t_(n))J₀e^(eV) ^(j)^(kT)>>kT/e), then a linear approximation of the current densitydistribution at the junction can be estimated according to the followingequationJ(x)=J ₁(e ^(−x/L) ^(s) +e ^(−(L-x)/L) ^(s) )

where J₁ is the current density beneath a pad, x is the distance from apad, and L_(s) is the current spreading length as shown in the followingequationL _(s)=√{square root over ((ρ_(p-c)+ρ_(n-c) /f+ρ _(p) t _(p)+ρ_(n) t_(n))t _(m)/ρ_(m))}

This estimation assumes an infinite separation between the pads.However, for a linear approximation with non-infinite separation, thesolutions for individual pads can be added together. The proceduredescribed above introduces an error close to the die center, but is notbelieved to significantly alter the physical trends.

The minimum current density can appear at the center of the device x=L/2and can be estimated according to the following the following equationJ _(min)=2J ₁ e ^(−L/2L) ^(s)

where the uniformity factor is estimated as shown in equation

$U = {\frac{J\left( {L/2} \right)}{J(0)} = {\frac{2\;{\mathbb{e}}^{{{- L}/2}L_{s}}}{1 + {\mathbb{e}}^{{- L}/L_{s}}}\;.}}$

For a die with the same surface area, switching from a square shape intoa rectangular shape with aspect ratios a,b where the contact bars arealong the small side, the minimum current density increases and theuniformity factor is modified as shown in the following equations

$\begin{matrix}{\mspace{11mu}{J_{\min}^{\prime} = {2J_{1}{\mathbb{e}}^{- \frac{\sqrt{A\;{b/a}}}{2L_{s}}}}}\mspace{169mu}} \\{{U^{\prime} = {\frac{J\left( {L^{\prime}/2} \right)}{J(0)} = \frac{2\;{\mathbb{e}}^{{{- \sqrt{A\;{b/a}}}/2}L_{s}}}{1 + {\mathbb{e}}^{{- \sqrt{A\;{b/a}}}/L_{s}}}}}\;}\end{matrix}$

-   -   Thus, a uniformity increase factor can be estimated as shown in        equation

$S = {{U^{\prime}/U} = {\frac{1 + {\mathbb{e}}^{{- \sqrt{A}}/L_{s}}}{1 + {\mathbb{e}}^{{- \sqrt{A\;{b/a}}}/L_{s}}}{\mathbb{e}}^{\frac{\sqrt{A}}{2L_{s}}{({1 - \sqrt{{b/a})}}}}}}$

For example, the uniformity increase factor ‘S’ has a minimum value S=1for the square case (e.g., a=b). For a 16×9 rectangle, assuming thefollowing values: ρ_(m)=2.2·10⁻⁶ Ωcm (gold), ρ_(p-c)=1.0·10⁻³ Ωcm²,ρ_(p)=5.0 Ωcm, ρ_(n-c)=1.0·10⁻⁴ Ωcm², ρ_(n)=5.0·10⁻³ Ωcm, n-contactsurface coverage 10%, and thicknesses for p-, n-, and metal 0.3 μm, 3.0μm and 2 μm (at a 10% coverage). Then L_(s) equals 1.4 mm. If the diehas a surface area of A=25 mm². In the square case U=0.325, while in the16×9 case U′=0.5, or a uniformity increase factor S=1.54, i.e. a 54%increase of current uniformity.

Thus, without wishing to be bound by theory, it is believed that using arectangular shape for an LED can provide benefits in the currentspreading. The contact resistivity can alternatively or additionally bealtered to enhance the current spreading by including an insulatinglayer 1820 (e.g., an oxide layer, FIG. 47A) underneath a portion of thecontact. As shown in FIGS. 47A and 47B, insulating layer 1820 (indicatedby dashed lines) is included under a portion of bar 1812. Insulatinglayer 1820 has a greater width at the top of the bar (e.g., close topads 1804) and gets thinner towards the central area of the die. Anequivalent circuit diagram is shown in FIG. 47B.

Contact resistivity is generally proportional to the contact area. Forexample, the contact resistivity increases as the contact area decreasesas shown in the following equation

$\rho_{n - c}^{eff} = {\frac{\rho_{n - c}}{f_{eff}} = {\frac{\rho_{n - c}W}{2w} = {\frac{\rho_{n - c}W\; L}{2x\; w_{b}} = {\frac{\rho_{n - c}}{f}\frac{L}{2x}}}}}$

where W is the repetition rate of the bars (e.g., the number of bars perunit area). Due to underlying insulating layer 1820, the area of thecontact is smaller at the edge of the contact closest to pads 1804 a and1804 b and increases as the distance from pads 1804 a and 1804 bincreases. Due to the difference in contact area, the contactresistivity is higher close to pads 1804 a and 1804 b and decreasesgradually towards the center of the LED. The difference in contactresistivity can force the current to travel further, reducing currentcrowding, increasing uniformity of light emission through the surface,and reducing performance degradation. The current spreading length canbe estimated according to the following equationL _(s)(x)=√{square root over ((ρ_(p-c)+(ρ_(n-c) /f)(L/2x)+ρ_(p) t_(p)+ρ_(n) t _(n))t _(m)/ρ_(m))}{square root over ((ρ_(p-c)+(ρ_(n-c)/f)(L/2x)+ρ_(p) t _(p)+ρ_(n) t _(n))t _(m)/ρ_(m))}.

The junction current density along the die can be estimated by thefollowing equation

J(x) = J₁ 𝕖^(−∫₀^(x) 𝕕x/L_(s)(x)) + J₁ 𝕖^(−∫_(L)^(x)𝕕x/L_(s)(L − x))  and

the minimum current is at the center of the device (e.g., at x=L/2) canbe estimated according to the following equation

J_(min) = 2J₁ 𝕖^(−∫₀^(L/2) 𝕕x/L_(s)(x))

The current uniformity factor for the structure shown in FIG. 47B can beestimated according to the following equation

$U = {\frac{J\left( {L/2} \right)}{J(0)} = {\frac{2\;{\mathbb{e}}^{- {\int_{0}^{L/2}\ {{{\mathbb{d}x}/2}{L_{s}{(x)}}}}}}{1 + {\mathbb{e}}^{- {\int_{0}^{L}\ {{{\mathbb{d}x}/2}{L_{s}{(x)}}}}}}\;.}}$

As described above, oxide layer 1820 can force current towards the endsof the contacts (e.g., toward the central area of the die) increasingthe current spreading. Oxide layer 1820 can also reduce the lightgeneration underneath the light absorbing contacts allowing greaterpercentage of the generated light to emerge from the surface of the LED.

FIGS. 48A and 48B show a further configuration of pads 1804 a and 1804b, contact 1830, and oxide layer 1820 (indicated by dashed lines anddisposed under a portion of contact 1830). Here, contacts 1830 are alsotapered. While shown in FIG. 48A as being linearly tapered, othertapering could be used. The linear tapering maintains a similar totalcontact area to the contact area of contact 1812 shown in FIG. 47A, withthe contact width at the die center being approximately half of thewidth of bars 1812 (FIG. 47A), while the contact width at the pads is 3times larger than the width shown in FIG. 47A. The oxide can be taperedat higher angle so that the contact resistance is maximum at the pad andminimum at the die center. The contact resistance decreases towards thedie center, and the bar resistance decreases closer to the pad. Thetapering of both the contact and the insulating layer contribute toforcing the current towards the die center. The local spreading lengthcan be estimated according to the following equation

${L_{s}(x)} = {\sqrt{\left( {\rho_{p - c} + {\left( {\rho_{n - c}/f} \right)\left( {L/x} \right)} + {\rho_{p}t_{p}} + {\rho_{n}t_{n}}} \right){t_{m}/\left( {2\;{\rho_{m}/\left( {3 - {4{x/L}}} \right)}} \right)}}.}$

Similar integration formulas for the current distribution as describedabove can be used to estimate the current distribution for the structureshown in FIGS. 48A and 48B.

FIG. 49A shows a top view and FIGS. 49B and 49C show cross-sectionalviews of an additional contact structure 1801. Conductive contacts 1836extend toward the center of the die, but do not continuously cover theupper surface of the LED between bars 1804 a and 1804 b. An insulatinglayer 1834 is located between the top of the LED and metal contact 1836in an interior portion of the contact. Both the contact 1836 and theinsulating layer 1834 are tapered. Arrows 1837 represent the currentspreading from the metal contact 1836 into the surface of the die.

FIG. 50 shows a graph 1850 of estimated normalized junction currentdensity as a function of the normalized distance between bars 1804 a and1804 b for various contact and die configurations based on the forgoingequations. Line 1856 represents the current density for square die withrectangular bars and no oxide, line 1858 represents the current densityfor rectangular die with rectangular bars and no oxide, line 1860represents the current density for a rectangular die with rectangularbars and tapered oxide, and line 1862 represents the current density forrectangular die with tapered bars and tapered oxide. Graph 1850 showsthe improvement in the current density distribution for both arectangular chip and an oxide layer under a portion of the contact.

FIG. 51A shows a top view and FIG. 51B shows a cross-sectional view ofan additional contact structure 1803. Insulating layers 1805 a and 1805b are located between the top of the LED and metal pads 1804 a and 1804b, respectively. Insulating layers 1805 a and 1805 b are located under aportion of metal pads 1804 a and 1804 b, respectively, toward the edgeof the die such that a portion of metal pads 1804 a and 1804 b aresupported by insulating layers 1805 a and 1805 b, respectively, and aportion of metal pads 1804 a and 1804 b are supported by the top surfaceof the light emitting diode. Oxide layers 1805 a and 1805 b reduce thelight generation underneath the light absorbing metal pads 1804 a and1804 b allowing greater percentage of the generated light to emerge fromthe surface of the LED.

While embodiments described above include a single set of contactsextending from metal pads 1804 a and 1804 b, multiple sets of contactscould be used. For example, a second set of contacts could extend fromthe set of contacts connected to metal pads 1804 and so forth. Further,while oxide layers have been described, most generally, the layers canbe formed of any appropriate electronically insulating material (e.g.,nitride).

FIG. 52 shows the dimensions of an example of a contact 1899 and can beused to estimate electrical transport inside the n-contact. It isassumed contact 1899 distributes a uniform current density J₀ withincontact period D 1870. The total current to be carried by the contactcan be estimated as shown in the following equationI _(max) =J ₀ DL.

This maximum current is flowing at the top of the contact (at the pad)corresponding to a current density that can be estimated as shown in thefollowing equation

$J_{\max} = {\frac{J_{0}D}{W\; T}L}$

At any distance x from the bar's end, the current density can beestimated as shown in the following equation

$J = {\frac{J_{0}D}{W\; T}x}$

The voltage drop per unit length can be estimated as shown in thefollowing equation

$\frac{\mathbb{d}V_{c}}{\mathbb{d}x} = \frac{J_{0}D\; R\; x}{W\; T}$

and the heat generated per unit length can be estimated as shown in thefollowing equation

$\frac{\mathbb{d}Q_{c}}{\mathbb{d}x} = \frac{2J_{0}^{2}D^{2}\; R\; x^{2}}{W\; T}$

Integrating the above equation the total voltage drop can be estimatedas shown in the following equation

$V_{c} = \frac{J_{0}D\; R\; L^{2}}{2W\; T}$

and the total heat generated in the bar can be estimated as shown in thefollowing equation

$Q_{c} = \frac{2J_{0}^{2}D^{2}\; R\; L^{3}}{3W\; T}$

When the total heat generated becomes significant, uniform currentassumption can break down, as can the device's performance (e.g., thedevice overheats). Therefore, it can be desirable to minimize themaximum current density (current density generally scales linearly withlength), the voltage drop (voltage drop generally scales with the squarelength), and/or the heat generated (heat generated generally scales withthe cube of the length). Based on the above relationships, a rectangular9×16 die having more but shorter bars has a, b and c reduced by a factorof 3/4, 9/16, and 27/64 respectively. Since the number of bars isincreased by a factor of 4/3, it is believed that the total heatgenerated can be reduced by a factor of 9/16.

FIG. 53 shows a packaged LED device 1890. In general, the package shouldbe capable of facilitating light collection while also providingmechanical and environmental protection of the die and allowing heatgenerated in the die to be dissipated. As described above, LED 1890includes conductive pads 1804 a and 1804 b that allow current to bespread to multiple contact fingers 1812 and dissipated to the LEDsurface. Multiple wire bonds 1892 provide an electrical current pathbetween the LED and the package. Wire bonds 1892 can be made of variousconductive materials such as gold, aluminum, silver, platinum, copper,and other metals or metal alloys. The package also includes multiplecastellations 1894 to transport current from a bottom surface of thepackage to a top surface of the package to facilitate surface mountingon a circuit board. Castellations 1894 include a central region and aplating layer. The central region can be composed of a refractory metal,for example, tungsten and can be relatively thick (e.g., about 100 um toabout 1 mm). The central region can be plated with an electricallyconductive material such as gold. The plating can range in thicknessfrom about 0.5 um to about 10 um and provides a current path thatsupports relatively high power levels. In addition, the package includesa transparent cover 1896 packaged on the LED die to protect thepatterned surface 506 (FIG. 36) when an encapsulant is not used. Thetransparent cover 1896 is attached to the package, for example, using aglassy frit that is melted in a furnace. Alternatively, cover 1896 canbe connected using a cap weld or an epoxy for example. The transparentcover 1896 can be further coated with one or more anti-reflectioncoatings to increase light transmission. Without wishing to be bound bytheory, it is believed that the absence of an encapsulant layer allowshigher tolerable power loads per unit area in the patterned surface LED100. Degradation of the encapsulant can be a common failure mechanismfor standard LEDs and is avoided not using an encapsulant layer.Packaged device 1890 can be mounted on a circuit board, on anotherdevice, or directly on a heat sink.

FIG. 54 shows a model of the heat dissipation for a packaged device 1890placed on a heat sink device. The packaged device 1890 is supported by acore board 1900 that includes insulating and electrically conductiveregions (e.g., conductive regions using metals such as Al or Cu)attached to the heat sink. For example, packaged device 1890 can beattached to core board 1900 using solder (examples of solder includeAuSn solder, PbSn solder, NiSn solder, InSn solder, InAgSn solder, andPbSnAg solder) or using an electrically conductive epoxy (e.g., silverfilled epoxy). Core board 1900 is supported by a layer of heat sinkmetal 1902 and heat sink fins 1904. For example, core board 1900 can beattached to heat sink metal 1902 using solder (examples of solderinclude AuSn solder, PbSn solder, NiSn solder, InSn solder, InAgSnsolder, and PbSnAg solder) or using epoxy (e.g., silver filled epoxy).In this model it is assumed that heat spreads from packaged device 1890as the heat dissipates towards the heat sink. Spreading angle 1906represents the angle at which heat dissipates out of packaged device1890. Spreading angle 1906 generally varies depending on the materialproperties and the vertical layout of the system. Spreading angle 1906can vary for different layers in the heat sink. The thermal resistanceof a slice with thickness d_(x) can be estimated according to thefollowing equation

${d\; R_{t\; h}} = {{\frac{d\; x}{K_{0}}\frac{1}{S_{x}^{''2}}} = {\frac{d\; x}{K_{0}}\frac{1}{\left( {S^{\prime} + {2x\;\tan\;\theta}} \right)^{2}}}}$

where K₀ is the thermal conductivity and S′ is the dimensions of theheat front at the top of the element. Integrating produces the followingequation for resistivity

$R = {\frac{d\;}{K_{0}}\frac{1}{S^{\prime}\left( {S^{\prime} + {2d\;\tan\;\theta}} \right)}}$

In the case of a rectangle, this resistivity can be calculatedgenerating the results shown in FIG. 55. FIG. 55 shows a calculatedratio of R_(th) _(—) _(rectangle)/R_(th) _(—) _(square) (where Rth isthe thermal resistance) for a system of large thickness and spreadingangle of 45°. As the aspect ratio increases, the thermal resistance candrop. For example, if a square die system has a thermal resistance of20° C./W and it is desired to dissipate 3 W of power, then the junctiontemperature (assuming an ambient temperature of 25° C.) can be25+20*3=85° C. A rectangular die of the same area and same dissipatedheat, however, will typically have a lower junction temperature. FIG. 56shows a graph of junction temperature as a function of aspect ratio. Itis believed that a lower junction temperature is desirable for reducedwavelength shift and higher device efficiency.

As described above, using a rectangular shape for an LED (compared, forexample, to a square) can provide certain advantages. The advantages caninclude one or more of the following. The rectangular LED can allow agreater number of wire bonds per unit area increasing the power that canbe input into the LED. The rectangular shape can be chosen to match aparticular aspect ratio of a pixel or microdisplay, thus, eliminatingthe need for complex beam shaping optics. The rectangular shape can alsoimprove heat dissipation from the LED and reduce the likelihood offailure due to the device overheating.

Because the cross section of an individual LEDs cut from a wafer is onlyslightly larger than the light-emitting surface area of the LED, manyindividual, and separately addressable LEDs can be packed closelytogether in an array. If one LED does not function (e.g., due to a largedefect), then it does not significant diminish the performance of thearray because the individual devices are closely packed.

While certain embodiments have been described, other embodiments arepossible.

As an example, while certain thickness for a light-emitting device andassociated layers are discussed above, other thicknesses are alsopossible. In general, the light-emitting device can have any desiredthickness, and the individual layers within the light-emitting devicecan have any desired thickness. Typically, the thicknesses of the layerswithin multi-layer stack 122 are chosen so as to increase the spatialoverlap of the optical modes with light-generating region 130, toincrease the output from light generated in region 130. Exemplarythicknesses for certain layers in a light-emitting device include thefollowing. In some embodiments, layer 134 can have a thickness of atleast about 100 nm (e.g., at least about 200 nm, at least about 300 nm,at least about 400 nm, at least about 500 nm) and/or at most about 10microns (e.g., at most about five microns, at most about three microns,at most about one micron). In certain embodiments, layer 128 has athickness of at least about 10 nm (e.g., at least about 25 nm, at leastabout 40 nm) and/or at most about one micron (e.g., at most about 500nm, at most about 100 nm). In some embodiments, layer 126 has athickness of at least about 10 nm (e.g., at least about 50 nm, at leastabout 100 nm) and/or at most about one micron (e.g., at most about 500nm, at most about 250 nm). In certain embodiments, light-generatingregion 130 has a thickness of at least about 10 nm (e.g., at least about25 nm, at least about 50 nm, at least about 100 nm) and/or at most about500 nm (e.g., at most about 250 nm, at most about 150 nm).

As an example, while a light-emitting diode has been described, otherlight-emitting devices having the above-described features (e.g.,patterns, processes) can be used. Such light-emitting devices includelasers and optical amplifiers.

As another example, while current spreading layer 132 has been describedas a separate layer from n-doped layer 134, in some embodiments, acurrent spreading layer can be integral with (e.g., a portion of) layer134. In such embodiments, the current spreading layer can be arelatively highly n-doped portion of layer 134 or a heterojunctionbetween (e.g. AlGaN/GaN) to form a 2D electron gas.

As a further example, while certain semiconductor materials have beendescribed, other semiconductor materials can also be used. In general,any semiconductor materials (e.g., III-V semiconductor materials,organic semiconductor materials, silicon) can be used that can be usedin a light-emitting device. Examples of other light-generating materialsinclude InGaAsP, AlInGaN, AlGaAs, InGaAlP. Organic light-emittingmaterials include small molecules such as aluminumtris-8-hydroxyquinoline (Alq₃) and conjugated polymers such aspoly[2-methoxy-5-(2-ethylhexyloxy)-1,4-vinylenephenylene] or MEH-PPV.

As an additional example, while large area LEDs have been described, theLEDs can also be small area LEDs (e.g., LEDs smaller than the standardabout 300 microns on edge).

As another example, while a dielectric function that varies spatiallyaccording to a pattern has been described in which the pattern is formedof holes, the pattern can also be formed in other ways. For example, apattern can be formed continuous veins and/or discontinuous veins in theappropriate layer. Further, the pattern in varying dielectric functioncan be achieved without using holes or veins. For example, materialshaving different dielectric functions can be patterned in theappropriate layer. Combinations of such patterns can also be used.

As a further example, while layer 126 has been described as being formedof silver, other materials can also be used. In some embodiments, layer126 is formed of a material that can reflect at least about 50% of lightgenerated by the light-generating region that impinges on the layer ofreflective material, the layer of reflective material being between thesupport and the multi-layer stack of materials. Examples of suchmaterials include distributed Bragg reflector stacks and various metalsand alloys, such as aluminum and aluminum-containing alloys.

As another example, support 120 can be formed of a variety of materials.Examples of materials from which support 120 can be formed includecopper, copper-tungsten, aluminum nitride, silicon carbide,beryllium-oxide, diamonds, TEC, and aluminum.

As an additional example, while layer 126 has been described as beingformed of a heat sink material, in some embodiments, a light-emittingdevice can include a separate layer (e.g., disposed between layer 126and submount 120) that serves as a heat sink. In such embodiments, layer126 may or may not be formed of a material that can serve as a heatsink.

As a further example, while the varying pattern in dielectric functionhas been described as extending into n-doped layer 134 only (which cansubstantially reduce the likelihood of surface recombination carrierlosses) in addition to making use of the entire light-generating region,in some embodiments, the varying pattern in dielectric function canextend beyond n-doped layer (e.g., into current spreading layer 132,light-generating region 130, and/or p-doped layer 128).

As another example, while embodiments have been described in which aircan be disposed between surface 110 can cover slip 140, in someembodiments materials other than, or in an addition to, air can bedisposed between surface 110 and cover slip 140. Generally, suchmaterials have an index of refraction of at least about one and lessthan about 1.5 (e.g., less than about 1.4, less than about 1.3, lessthan about 1.2, less than about 1.1). Examples of such materials includenitrogen, air, or some higher thermal conductivity gas. In suchembodiments, surface 110 may or may not be patterned. For example,surface 110 may be non-patterned but may be roughened (i.e., havingrandomly distributed features of various sizes and shapes less thanλ/5).

As another example, while embodiments involving the deposition andetching of planarization and lithography layers have been described, insome embodiments, a pre-patterned etch mask can be laid down on thesurface of the n-doped semiconductor layer.

As a further example, in some embodiments, an etch mask layer can bedisposed between the n-doped semiconductor layer and the planarizationlayer. In such embodiments, the method can include removing at least aportion of the etch mask layer (e.g., to form a pattern in the etch stoplayer corresponding to the pattern in the n-doped semiconductor layer).

As an additional example, while embodiments, have been disclosed inwhich surface 110 is patterned and smooth, in some embodiments, surface110 may be patterned and rough (i.e., having randomly distributedfeatures of various sizes and shapes less than λ/5, less than λ/2, lessthan λ). Further, in certain embodiments, the sidewalls of openings 150can be rough (i.e., having randomly distributed features of varioussizes and shapes less than λ/5, less than λ/2, less than λ), with orwithout surface 110 being rough. Moreover, in some embodiments, thebottom surface of openings 150 can be rough (i.e., having randomlydistributed features of various sizes and shapes less than λ/5, lessthan λ/2, less than λ). Surface 110, the sidewalls of openings 150,and/or the bottom surfaces of openings 150 can be roughened, forexample, by etching (e.g., wet etching, dry etching, reactive ionetching). Without wishing to be bound by theory, it is believed thatroughening surface 110 and/or the sidewalls of openings 150 may increasethe probability, with respect to a atomically smooth surface, that alight ray will eventually strike at an angle that less than the criticalangle given by Snell's law and will be extracted.

As another example, in some embodiments, the submount can be machined toinclude spring-like structures. Without wishing to be bound by theory,it is believed that such spring-like structures may reduce crackingduring removal of the substrate.

As a further example, in some embodiments, the submount can be supportedby an acoustically absorbing platform (e.g., a polymer, a metallicfoam). Without wishing to be bound by theory, it is believed that suchacoustically absorbing structures may reduce cracking during removal ofthe substrate.

As an additional example, in some embodiments, the substrate is treated(e.g., etched, ground, sandblasted) before being removed. In certainembodiments, the substrate may be patterned before it is removed. Insome embodiments, the thickness of the layers is selected so that,before removing the substrate and buffer layers, the neutral mechanicalaxis of the multi-layer stack is located substantially close (e.g., lessthan about 500 microns, less than about 100 microns, less than about 10microns, less than about five microns) to the interface between thep-doped semiconductor layer and a bonding layer. In certain embodiments,portions of the substrate are separately removed (e.g., to reduce thelikelihood of cracking).

As another example, while embodiments have been described in which abuffer layer is separate from an n-doped semiconductor layer (e.g., abuffer layer grown on the substrate, with an n-doped semiconductor layerseparately grown on the buffer), in some embodiments, there can be asingle layer instead. For example, the single layer can be formed byfirst depositing a relatively low doped (e.g., undoped) semiconductormaterial on the substrate, followed by (in one process) depositing arelatively high doped (n-doped) semiconductor material.

As a further example, while embodiments have been described in which asubstrate is removed by a process that includes exposing a surface ofthe substrate to electromagnetic radiation (e.g., laser light), in someembodiments other methods can be used to remove the substrate. Forexample, removal of the substrate can involve etching and/or lapping thesubstrate. In certain embodiments, the substrate can be etched and/orlapped, and then subsequently exposed to electromagnetic radiation(e.g., laser light).

As an additional example, in some embodiments, after depositing theplanarization layer but before depositing the lithography layer, theupper surface of the planarization layer can be flattened. For example,a flat object, such as an optical flat, can be placed on the uppersurface of the planarization layer while heating the planarization layer(e.g., with a hot plate). In some embodiments, a pressure can be applied(e.g., using a physical weight or press) to assist with the flatteningprocess.

As another example, in some embodiments the substrate can be treatedbefore being removed. For example, the substrate can be exposed to oneor more processes selected from etching, polishing, grinding, andsandblasting. In certain embodiments, treating the substrate can includepatterning the substrate. In some embodiments, treating the substrateincludes depositing an antireflective coating on the substrate. Such anantireflective coating can, for example, allow relatively large regionsof the substrate to be removed when using a substrate removal processthat involves exposing the substrate to electromagnetic radiationbecause the coating can reduce reflection of the electromagneticradiation. In certain embodiments, a pattern on the surface of thesubstrate can also be used to achieve an anti-reflection effect.

In some embodiments, a light-emitting device can include a layer of aphosphor material coated on surface 110, cover layer 140 and supports142.

In certain embodiments, a light-emitting device can include a coverlayer 140 that has a phosphor material disposed therein. In suchembodiments, surface 110 may or may not be patterned.

In an alternative implementation, the light emitted by thelight-generating region 130 is UV (or violet, or blue) and the phosphorlayer 180 includes a mixture of a red phosphor material (e.g.,L₂O₂S:Eu³⁺), a green phosphor material (e.g, ZnS:Cu,Al,Mn), and bluephosphor material (e.g, (Sr,Ca,Ba,Mg)₁₀(PO₄)₆Cl:Eu²⁺).

Other embodiments are in the claims.

1. An optical display system, comprising: a microdisplay having asurface; and a light emitting device, the light emitting deviceincluding: a multi-layer stack of materials including a light generatingregion, and a first layer supported by the light generating region, asurface of the first layer being configured so that the light generatedby the light generating region can emerge from the light emitting devicevia the surface of the first layer; wherein a ratio of an aspect ratioof the surface of the microdisplay and an aspect ratio of the surface ofthe first layer is from about 0.5 to about two, and wherein the lightgenerated by the light emitting device illuminates the surface of themicrodisplay and the microdisplay is configured to project an image. 2.The optical display system of claim 1, wherein the ratio of the aspectratio of the surface of the microdisplay and the aspect ratio of thesurface of the first layer is from about 9/16 to about 16/9.
 3. Theoptical display system of claim 1, wherein the ratio of the aspect ratioof the surface of the microdisplay and the aspect ratio of the surfaceof the first layer is from about 3/4 to about 4/3.
 4. The opticaldisplay system of claim 1, further comprising at least one opticalcomponent disposed between the light emitting device and themicrodisplay.
 5. The optical display system of claim 4, wherein the atleast one optical component is a lens.
 6. The optical display system ofclaim 1, comprising a plurality of light emitting diodes.
 7. The opticaldisplay system of claim 1, wherein the aspect ratio of the microdisplayis selected from a group consisting of 640×480, 800×600, 1024×700,1024×768, 1024×720, 1280×720, 1280×768, 1280×960, 1920×1080 and1280×1064.
 8. The optical display system of claim 1, wherein a shape ofthe surface of the microdisplay is rectangular, circular, trapezoidal,triangular, square, elliptical, or hexagonal.
 9. The optical displaysystem of claim 8, wherein a shape of the surface of the first layer isrectangular, circular, trapezoidal, triangular, square, elliptical, orhexagonal.
 10. The optical display system of claim 1, wherein a shape ofthe surface of the first layer is rectangular, circular, trapezoidal,triangular, square, elliptical, or hexagonal.
 11. The optical displaysystem of claim 1, wherein a shape of the surface of the microdisplay isrectangular and a shape of the surface of the first layer isrectangular.
 12. The optical display system of claim 1, wherein a shapeof the surface of the microdisplay is square and a shape of the surfaceof the first layer is square.
 13. The optical display system of claim 1,wherein the light emitting device is a non-lambertian light emittingdevice.
 14. The optical display system of claim 1, wherein the lightemitting device is a photonic lattice light emitting device.
 15. Theoptical display system of claim 1, wherein the surface of the firstlayer has a dielectric function that varies spatially according to apattern, and the pattern has an ideal lattice constant and a detuningparameter with a value greater than zero.
 16. The optical display systemof claim 1, wherein the surface of the first layer has a dielectricfunction that varies spatially according to a nonperiodic pattern. 17.The optical display system of claim 1, wherein the surface of the firstlayer has a dielectric function that varies spatially according to acomplex periodic pattern.
 18. The optical display system of claim 1,wherein the light emitting device is selected from the group consistingof light emitting diodes, lasers, optical amplifiers, and combinationsthereof.
 19. The optical display system of claim 1, wherein the lightemitting device comprises a light emitting diode.
 20. The opticaldisplay system of claim 1, wherein the light emitting device is selectedfrom the group consisting of OLEDs, flat surface-emitting LEDs, HBLEDs,and combinations thereof.
 21. The optical display system of claim 1,wherein a shape of the light emerging from the light emitting device issubstantially the same as a shape of the microdisplay.
 22. The opticaldisplay system of claim 1, further comprising a contact disposed on anarea around a perimeter of the surface of the first layer.
 23. Theoptical display system of claim 22, wherein the contact area is matchedto an area outside an area imaged on the microdisplay by the lightemitting device.
 24. The optical display system of claim 1, wherein theratio of the aspect ratio of the surface of the microdisplay and theaspect ratio of the surface of the first layer is about one.
 25. Theoptical display system of claim 1, wherein the optical display system isconfigured to be used in a rear projector.
 26. The optical displaysystem of claim 1, wherein the optical display system is configured tobe used in a rear projection television.
 27. The optical display systemof claim 1, wherein the optical display system is configured to be usedin a front projector.
 28. The optical display system of claim 1, whereinthe optical display system is configured to be used in a home theatresystem.
 29. The optical display system of claim 1, wherein the opticaldisplay system is configured to be used in a portable projector.
 30. Theoptical display system of claim 4, wherein the microdisplay, the lightemitting device and the optical component are positioned so that, duringuse, an image plane of the system does not coincide with a surface ofthe microdisplay illuminated by light emitted by the light emittingdevice.